I have two G3's. I've used them some on bass, but I'm not a huge effects user, mainly just messing around, but here's my experience.
Effects work great. Distortions and amp models aren't so great, they lose a lot of low end.
That's pretty much it. If you want a bunch of good delays, choruses, verbs, etc, and don't care about drives, the G series are good. If you want overdrives, get the B3.