The bite depends on the species, age and other factors.
In the case of the bimac, most people experience only a curious little nibble, not as bad as a bee sting. I don't think anyone has reported even being nibbled on by a briareus. (Please respond if you have!)
The dwarfs can be agressive and seem to have more toxin in their bites. I belive it was Greg (cthulhu77) who reported pain for several hours. Larger octopuses have larger beaks and therefore larger bites, but many are not aggressive. Neogondactylus mentioned O. cyanea as a biter.
A few are defensive biters, like O. rubescens. Still, I watched someone handling small O. rubescens (mantle about the size of an orange) and he recieved an "ouch" type bite, but also not too bad.
It was not a real attack.
The thing to watch for is an allergic reaction to the toxin, like that for a beesting. I don't think you should encourage octo nibbling or biting, but if it happens, you'll most likely survive.