I've owned the ME-20, ME-50 and ME-50B. I don't own them anymore, though I'm reconsidering as I've been going through pedal flux lately.
These Boss units are decent. Not great, but okay. They do a few things very well, and lots of other things below par. Lots of gimmicky stuff like super-oscillator-polyoctave-alien spaceship sounds. You're getting a convenient form factor at a fair price, but I don't think the sound quality rivals good individual pedals that I've tried.
I think you need to try one in YOUR amp, because the type of speaker/cab you use really makes a difference in how they sound. Multi-fx units with amp simulation are often geared to full-range, flat response systems (like a keyboard amp or PA), but in this case you can defeat the amp sims and just use it in individual pedal mode into a guitar amp. So it comes down to whether or not these particular pedal emulations give you what you want when paired with your other gear.
If you're looking only for a distortion pedal, get a good distortion pedal.
But if you need a programmable multi-fx unit and want to use your own guitar amp, then the ME-70 is probably a good option. I'd also compare it to the Digitech RP500 or RP1000; they've really improved over the years and these units are very competitive with the Boss systems. Haven't tried the Line6 M9, but that one is getting good marks too.