Here is a cut/paste from a post I did some time back:
------------------------------------------------------
Article worth reading: http://fwp.mt.gov/mtoutdoors/downloads/BearAttack.pdf

Another article worth reading: http://www.nps.gov/glac/parknews/news10-05.htm

Best information I've read on Bear Spray vs. Firearms on bears:

"Between 2005 and 2009, park visitation totaled 9,835,188. During that timeframe, three visitors were injured by grizzly bears in Glacier. Bear spray was not used by any of those three individuals. Glacier managers agree with Montana Fish, Wildlife, and Parks’ statement: "If you are armed, use a firearm only as a last resort. Wounding a bear, even with a large caliber gun, can put you in far greater danger."

"According to U.S. Fish and Wildlife Service (USFWS) investigations of human-bear encounters since 1992, persons encountering grizzlies and defending themselves with firearms suffered injury about 50% of the time. During the same period, persons defending themselves with pepper spray escaped injury most of the time and those that were injured experienced shorter duration attacks and less severe injuries.

"Other researchers have come to the same conclusions. According to the USFWS, Canadian bear biologist / bear conflict expert Dr. Stephen Herrero, a person’s chance of incurring serious injury from a charging grizzly doubles when bullets are fired versus when bear spray is used. Also, in a study published in the April 2008 Journal of Wildlife Management, Tom Smith examined "The Efficacy of Bear Deterrent Spray in Alaska." The study showed that in 72 cases where people used bear pepper spray to defend themselves from bears, the spray stopped bears 92 percent of the time and 98 percent of the people involved were uninjured."