Never seen bear spray in use but I've seen O.C. spray used on plenty of humans including myself. Oleoresin capsicum, or pepper extract, is the active ingredient used in both. It can be very effective if (A) you manage to spray it in the general vicinity of the eyes, nose or mouth and (B) you manage to deliver most of the product on the target and not on yourself or your allies. Easier said than done. And that's assuming you can unholster it, pull the safety pin, and accurately deliver an effective dose in a timely manner, which could be mere seconds.

Not all humans are affected by O.C.spray even when accurately applied and I can only assume that bears are no different.

Good article on Pepper Spray vs Griz.