In answer to the original question, I think the bear spray is more effective than most handguns - definitely a .357 Magnum. The real big bore hand cannons are the exception, but they are quite a load to lug around all day. If I am seriously concerned about the issue (like hunting in Alaska), I (or someone in my group) would probably be packing a short barrel 12 guage as well as some bear spray.

I have personally seen the bear spray deployed. One of my fellow park rangers and I encoutered a medium size black bear inside a5 yard dumpster once, feeding on garbage. The other ranger gave him a shot of the OC spray and he came out of that dumpster so violently that his claws creased 1/8" steel and actually tipped over the container. He ran right into a tree, made a course correction, and left the area in a death rattle panic.