The Boss ME-50 comes in a rugged metal housing, about the size of 4 pedals put together. The build is solid, looks and feels roadworthy so that is a definite plus. The unit has 2 inputs on the back - one is 1/4 mono input for guitar and a 1/8 stereo jack for a jam-along device such as an mp3 player, CD player, etc. On the back there are 3 outputs - one pair amplifier outs (L and R output, the left doubling as mono), and a 1/4 inch stereo headphone output. There is an input jack for an external Boss FS-5U switch or 2 when using a Boss PCS-31 cable on the back as well. There are 3 stomp type pedals and one expression pedal on the top of the pedal, which can be controlled by foot.
The ME-50 could be powered by Boss PSA-120 power supply (not included) or 6 AA batteries.
The test unit arrived with a paper manual (thank you Boss!) and 6AA batteries. The manual is quite straightforward and at about 45 pages covers pretty much everything you need to know about this pedal.
The effect section of the consists of Tone Modify, Expression, Compressor, Overdrive/Distortion, Noise Suppressor, Volume Pedal, Modulation, Delay and Reverb.
Tone Modify changes the characteristics of the connected guitar. It can be bypassed or when engaged work as what I would probably call an eq section and an exciter section. Its settings are:
-Off (effect bypassed)
-Fat (fat tone with boosted mid range)
-Presence (bright tone with boosted high mid-range)
-Mild (mild tone with high end rolloff)
-Tight (tone with low frequency cut)
-Enhance (tone with high frequencies boosted)
-S->H (single pickup tone is changed to humbucker sound)
-H->S (humbucker sound is changed to single coil mode)
-H->HF (changes humbucking tone to a single coil pickup half tone)
-Hollow (adds body resonance to create a full acoustic guitar sound)
-Acoustic (changes guitar tone to that of acoustic guitar)

The ME-50's Tone Modify section can be switched on/off via an external footswitch (not provided).

The Compressor comes with two knobs - Sustain and Level. The Sustain knob adjusts the depth of compression. The Level knob adjusts the volume level. The compressor can also be used as a limiter when the Sustain knob is to the left of the 12 o'clock position. This function could also be switched via an optional external footswitch.

The Overdrive/Distortion consists of Type selector switch, a Variation push switch, and 4 rotary switches - Drive, Bottom, Tone, and Level. The Type switch selects the distortion types, 22 types in total:

OD-1 - models Boss OD-1
OD-2 - models Boss OD-2
BD-2 - Models Boss BD-2
DS-1 - Models Boss DS-1
SCREAM - models Ibanez Tubescreamer TS-808
DST+ - models MXR distortion +
GUV - Models Marshall Guvnor overdrive
RAT- Models ProCo Rat distortion
MUFF - models Electro-Harmonic Big Muff Pi
FACE - Models the Fuzzface
MT-2 - models the Boss Metalzone II distortion

metal - metal distortion
loud - heavy distortion with boosted low end
lead - smooth lead distortion
crunch - mild overdrive usually heard in classic rock
natural - smooth bluesy type of distortion
modern OD - overdrive with boost in the midrange
stack - sound of a full stack distortion
hi gain - overdrive through a stack amp
modern DS - large high gain amp distortion
square - synth square wave sound
oct fuzz - fuzz sound produced by octave harmonics

The Variation knob select the variation which give the user a choice 22 distortion settings in total. The Drive knob dials in the amount of distortion, the Bottom knob acts as low eq, the Tone knob acts as high eq while the Volume knob adjusts volume.

The Modulation section is also similarly laid out. The Type button select the types of modulation effects - from chorus to rotary speaker. It comes with 3 knobs - Rate/Key, Depth/Harmony and E. Level/Resonance which have different functions depending on the effect. The Modulation section is capable of syncing an effect to a given tempo.

The Delay section consists of Type selector which chooses the kind of delay, Time the lenght, Feedback the amount of feedback introduced into the signal and E.Level the effect level. As the Modulation section, a delay can be synced to tempo as well.
The Noise Suppressor (NS) is a single knob with a LED indicator showing when it is activated.

The Reverb pedal also has one control which selects between off mode and 4 modes of reverb - Room, Hall, Spring, and Mod.

The Expression Pedal usually works as a volume pedal unless when strongly pressed it activates the expression mode switch. The expression modes available are Wah (wah pedal), Resonance (synth filters), Voice ("talking" modulation akin to a human voice), Ring Modulation (metallic, robotic sounds), +1 Octave (raises pitch up by octave) and -1 Octave (lowers pitch by octave).

The ME-50 uses two modes - Manual and Memory Mode.
Memory Mode allows the user to store and call up presets, 3 presets per bank, there are 10 banks so the user can have 30 presets in total.
Manual mode makes the ME-50 function in pedal mode - basically the three pedals become on/off for distortion, modulation and delay, and the foot controller functions as a volume control or expression pedal, depending on the user selection.
The user can navigate between memory and manual mode by pressing pedal 2 and 3 together.
The ME-50 also comes in with a chromatic tuner which is activated when pressing pedal 1 and 2 together. Engaging the tuner mutes the signal output so this function is quite useful for tuning live. Reference pitch could be easily changed as well in a range between 435 to 445 hz.
At the end of the signal chain the ME-50 has a Master volume control which controls the overall volume going out of the unit.

In use
Right away I was impressed by the "analog" sound of this multi-fx unit. What I can say is that most of the all-in-one effects pedals that I've tried don't sound as organic as this box. What I also really liked about this unit is the fact that it can be powered by batteries, so it can be taken to places where there's no electricity. I wish I had one of these on my camping trips when I was a kid!

The thing that became apparent from the start was that this unit doesn't have a dedicated eq section, which seemed really strange for me, considering that amount of effects and parameters that can be tweaked. Basically the eq section seems to be the "Tone Modify" which also worked as an enhancer and acoustic simulator.

Usually a big letdown in the multifx category are the distortion sounds and unlike some other units that I've tried, the ME-50 actually had quite a few distortion/overdrive options and most of them sounded good. I feel that the distortion section has quite a lot of variety which will usually keep most musicians happy for a long time. Most of the time I found myself gravitating towards the OD-2, Scream and Guv distortion settings. Having recently tested the ProCo Rat I also found that Boss did quite a good job on emulating it. On the other hand, I found the MT-2, a staple of Boss' pedal division, to sound uninspired when compared to the original - this one sounded a bit overcooked and didn't have the amount of control in the eq section that usually help me to zero in on the offending frequencies. The Overdrive/Distortion section has 4 parameter controls - Drive, Bottom, Tone, Level. The Bottom control acts as a Bass eq, while the Tone eq's the high end. This made up somewhat for the lack of dedicated eq control to the whole unit, but can be used only when this effects section is active.

The presets on these types of units are usually over the top to show the maximum capacity of the pedal, so after going through the banks just seeing what is in store I started twiddling on my own. This unit is really easy and intuitive to operate - basically you go into "Edit" mode and start twisting knobs until you get what you like. I didn't really have to touch the manual for any of it, which again is a very high mark from me. When you're happy with the sound you got you commit the patch to memory by pressing the "Write" pedal once, then selecting the patch number with the bank up and down button and the pedals (1,2,3) lets you navigate to the patch you want to overwrite and pressing "Write" again commits the patch to that memory slot. On the other hand - just pressing "Write" twice commits the patch to the last sound you started working from.

I found the modulation effects on the ME-50 to be quite capable of producing a wide range of sounds, all the effects in this section were top notch and I couldn't ask for anything better except maybe for more control over the harmonizer and a possibility to use more than one effect from here - for example you can't run a Chorus and Harmonizer at the same time. Not a deal breaker as I usually don't but a little extra flexibility could've been useful for some players looking to achieve some extra weirdness.

The Delays on this unit are also quite capable of producing great results - there are plenty of options here and they all sounded good. I found myself using mostly the Analog delay setting which had a very nice warmth to it. This unit also has some really nice stereo delay settings - especially the PAN and SPACE PAN settings which have some great psychedelic sounds.

The noise gate and compression on this pedal were a bit of a let down. I felt that there was definitely not enough control to either one of them and I preferred to keep them off for most of the patches unless really necessary. The NS (noise gate) usually didn't clamp down fast enough or as aggressively as needed on some metal patches and just left some nasty noise artifacts trailing after the sound. The Compressor also felt as it was either on or off, there were no subtle settings.

The Reverb settings on the ME-50 were adequate, but again I felt that the unit skimped a bit on these features. There are 4 settings on one dial - Room, Hall, Spring and Mod. The spring reverb was wobbly and non-musical, the rest were adequate but didn't feel very responsive.

Overall I was able to dial in some quite capable sounds and found some great classic rock, metal and shred sounds on this unit. The acoustic simulator was also quite decent and in a bind I wouldn't mind using it live instead of switching guitars. I had a hard time getting sound levels for clean guitar sounds as the Overdrive/Distortion section for these was bypassed, so I had to pump up some compression level to get closer in volume level.

I also found that I usually had problems with the amount of patches I could access - 3 patches per song weren't always enough so I took out a keyboard pedal I had laying about, attached it to the external expression pedal and that provided patch up capability for this unit. With the PCS-31 and two Boss pedals one could have full patch up and down capability live which increases the cost of the pedal by about $80 and its footprint as well, but it is there if needed. The ME-50 also had some lag in switching which produced a few miliseconds of no sound before switching the tones in memory mode (check mp3s a sound file of this). Boss also confirmed the problem saying that "changing DSP and analog audio signal paths, as well as patch parameters can sometimes cause a small delay in patch changing".When in manual mode this problem wasn't present, and from what I am made aware of most people buy this unit to use as separate effects. I've heard this lag on some other brands of multifx units so you'd have to decide for yourself if this is is a deal breaker. It is also worth noting that this unit does not have midi capabilities, which at this price range is not a huge issue, but it would've been a nice touch.

The Modulation and Delay settings can be locked to tempo on the fly when playing live by holding the corresponding pedal so I found that very useful to dial in chorus or delay effects on the fly. The foot controller can also be used live as Volume control and when pressed hard the selected expression patch, say wah is activated. I found that quite useful live as you can turn it on and off as desired and still have the volume control available at my feet.

Overall I found the ME-50 to be a quite capable unit and it should sit well among its competition.