Both Spike and Oz will tolerate having their bellies rubbed for a few seconds, but will eventually start with the bunny-kicks and the biting. J's pretty bad -- he'll ignore their "stop rubbing my tummy!" signals and get them all riled up. Me, I don't like being bitten or clawed, so as soon as they start to show signs that they're getting tired of the belly-rubs, I stop.
Oz has this thing that he does where, if you rub his tummy, he'll wrap his paws around your wrist and gently bite down. If you pull away, he'll bite harder, but if you relax he'll let go.
Both of them have very rub-able tummies, though.
Oz has a big golden belly and Spike has flashes of white on his tummy -- they're hard to resist!