Once an animal has decided to attack, pepper spray is really useless.
This is flat not true. Pepper spray has been used successfully on numerous occasions to ward off bear attacks.
The myth that pepper spray doesn't work is quite popular among a certain segment of the population, but a recent analysis of 83 incidents of use of bear spray (
"Efficacy of Bear Deterrent Spray in Alaska") showed it to be highly effective.
See also
Rick Sinnot's article for more on popular myths regarding bears. Sinnot is a bear biologist recently retired from the Alaska Department of Fish and Game. Over the years he has investigated numerous bear-human encounters in Alaska.