It's a digital unit with COSM effects. I bought it three months ago, at first, strictly for its compressor. That section has several great compressors for recording, but I like the Limiter choice best for live work.
I was careful to A/B my tone with and without the effects unit (I plug into the effect and from that into the amp input - I don't use the effects loop) and I can't tell that my P-bass with flats is losing anything just because of being plugged into the unit. I also tried a Jazz with rounds; same thing. (I don't go for a modern, active tone, so there very well could be some loss that would affect those players.)
I also started with a neutral patch and added limiting to taste, then copied this patch across several banks as a starting point for my overdrives/fuzzes/distortions/etc. I really only used EQ on some of them; I use my amp EQ mostly, and for recording I may do some with the mixer.
I like how the overdrives/dist let you blend in some clean tone. Since all my patches are clones of the same 'perfect' tone, all my dirty patches share the same foundation, and thus are really consistent when switching live.
There are limitations as to what effects you can use at once - that might be a big turnoff - but I'd say that either in "memory mode" (which I use), or "stompbox mode" (also fun and useful), the ME-50B is great for what I do.
If you want to hear it in a recording, I have two songs on my Facebook Music Player, right toward the top of the list:
http://www.facebook.com/chriswilliamsmusician
"Face The World (2010 remake)" is Jazz Bass through a 60s sounding preset (the bass is pretty present and gets more active after the first chorus...)
"Day To Day (2010 remake)" is P-Bass through the same preset, but EQ'd differently in the mix
I wrote both of these songs in 1991, so forgive that songwriting.

I redid these for an old friend who wanted to hear them again, and I wouldn't un-vault the originals
If you want to hear the ME-50B in more of a rock setting, listen to "Obligations (2010 remake)"
And one of the ME-50B overdrive patches I made is used prominently in "Screwface (2010 remake)" - then switches to a fatter reggae tone.