There's a show that's been airing this week on the National Geographic channel all about bears -- Polar bears, Grizzly bears and Black bears. If I remember the statistics correctly, a bear's sense of smell is something like a hundred times more acute than a dog's (which I think they said is 7 times more acute than humans... whatever the numbers, a bear's sense of smell is it's greatest asset). It stands to reason that Bear Spray would be far more effective than just about anything else.
The show tells a story of two guys who got surprised by an aggressive Grizzly and tried to climb a tree to get away from it. In his panic trying to get away, one guy drops his vest which had pepper spray in it. The bear followed him up the tree and dragged him down. The guy falls about 30 feet, gets up and makes a run for his vest. He gets the pepper spray out and gives the bear a full-face shot of the stuff then turns and heads up another tree. He said the bear dropped to the ground at the base of the tree and lay there in a heap. The guy thought he'd killed the bear and he just watched it for a while. The bear finally got up, shook his head, snorted like he was trying to clear his sinuses, and ambled off without looking back for either of the two guys.
Cool show... very informative.