It is very effective.  But only if you know how to use it correctly.  I would suggest taking a bear awareness class.  You'll learn lots of valuable information.  If you live in Anchorage area, Learn to Return offers very informative classes with instructors that are fun to learn from.  I'm sure there are other classes offered, too.  I think I saw something about a bear symposium coming up in the Anchorage area and they will be teaching people how to use bear spray.