I have an ME50. I got it for three main reasons.
1) Guitar is not my main axe - so I have not accumulated a collection of stomp boxes (this gave me several effects "pedal" options in one box).
2) I did not want or need amp modeling (since I already own a POD, J-Station and a few combo's)
3) I wanted something with knobs (vs. multiple windows/screens)
Since I've not owned the pedals
the ME50 simulates - I can't testify how accurate the sounds are, but I can get close to any sound I'm trying for (very easy & fast - because I can tweak knobs).
I do find the ME50 works real well in front of a Fender Champ and a Roland Cube (pretty much set up an count to 4) and with some work it works with a small Marshall I own. The ME50 does not work as well in front of the POD or J-Station) - but that may simply be a reflection my ability (or lack thereof).
I don't gig as a guitar player (but I do sit in a various jams) - and the ME50 can take plenty of abuse - it's built like a tank. It's fairly heavy to haul around (vs. one or two pedals) - but that is a fair trade off for all it can do.
I don't know how it compares to all the others multi effects units (I didn't try many others) but for the price and the options it provides, all in all I'm satisfied with it.