I own, and have gigged for years with the ME50B. I know this term is overused, but it really is built like a tank. At practice (2 different bands), I play through poor p.a. systems and am able to dial in a decent tone very quickly. Let me list what I feel are the best features:
Rugged!
Built in tuner
Volume/expression pedal
Compressor
Easy, stompbox style interface.
And the drawbacks:
Memory mode lag time. When switching between saved presets, their is a 1/4 to 1/2 second lag. It's not like simply switching pedals on. If you won't be tapdancing during a song, this is a non-issue.
Lame synth sounds. I can't for the life of me dial in anything remotely usable, and when I have, I certainly couldn't recreate it a second time. But this is not the type of unit you would be using in most situations that require a synth sound.
I leave this unit in the basic stompbox mode with a little compression, sonic enhancer, natural tube growl, chorus/reverb and the expression pedal set to resonance filter (which gives a better filter sweep than the wah). Having a volume pedal is priceless.
I would totally recommend this unit to anyone at the beginner to intermediate level as it will introduce you to a world of effects and is rugged enough to last for years.
As a bonus, with enough knob twisting, I can get my baritone guitar to sound like a les paul through a marshal stack, even through my bass amp. I would totally buy this again if it were lost or stolen.