I've been using the M80 that I got off Donny for a while now. I really like it, the EQ is voiced really well, and it's pretty usable. The distortion channel sounds pretty good too, and the clean blend is really useful.
It can't really cop that sansamp growl though (from what I've gathered, I've never had one to compare). It has it's own vibe, and I've used it live and to record, but if you want that nice grit, a sansamp is probably more your speed.
One thing to consider is that with a sansamp, that characteristic sound is always there. When I was doing research on the pedals, most people seemed to like the MXR because it didn't have that characteristic color to it.
I do want to get a sansamp paradriver at some point (pedal form of the RBI), just because it's got a different flavor, but for now, the MXR suits me fine. All depends on what you want tonally. Like John said, it really is just personal preference.