I've seen a lot of bears, both grizzlies and black bears. I saw two grizzlies over easter weekend - a photo of one is attached:
It never ceases to amaze me how stupid many people act when they are around bears. What is truly amazing is that we don't read about more attacks. If anything, the bears are incredibly tolerant of us. If you look at the http://www.udap.com testimonials, there is one from glacier park with a black bear that chased some kids under their vehicle. The testimonial was mine, as are the photos. I don't know what set that bear off, as we came across the event after the bear already started following these folks, but the bear spray worked well on that bear, which unfortunately was later destroyed. That is the only time I have seen a bear do more than grunt or stamp its paws at someone. Generally, they don't want to be around you at all - so in bear country make a lot of noise, the more obnoxious the better. My singing, and that of my wife and kid has worked well to clear the trail.
Also, I saw one previous poster indicated he would use a .380 as backup to a can of bear spray after an incident in Yellowstone. Others commented on using a firearm for bear protection. Firearms are illegal in Yellowstone, Glacier and other national parks, so that really isn't an option there anyway. Second, if you've seen a bear really move, you know you'd never get the chance to use your spray and then a gun. They are incredibly fast. Even when you have a gun, chances are you'll not be prepared to use it on the bear.
Bears are just plain incredible animals. My thoughts are that for anyone in bear country, the best thing for you and the bears is to make so much noise you don't see any bears, and if you do, just plain leave them alone.