I've been messing around with an MXR M82 for about a week and just got a Groove Regulator delivered today. Admittedly, I have much more time with the M82, but as much as I expected to love the Groove Regulator, I think I dig the M82 more.
M82 is pretty easy to dial in a nice subtle effect, particularly with the ability to mix dry signal back in. It also just seems more fine tuned for bass. Even when I get the GR tweaked close to the way I like it, the effect seems more sensitive to my E string than my other strings. Odd because the exact same input into the M82 has a much more even sensitivity across the strings. And the huge advantage of the M82 for me is that I can get a usable finger and slap sound out of it at a single sensitivity setting. This seems out of the question so far with the GR.
Admittedly, the M82 pretty much has one sound, while you can get all sorts of funky sounds out of the GR, but for one set it and forget it effect for me, it looks like the M82.