I've got "Counter Assault" bear deterrent. I've used it twice, once on an aggressive dog (he's not aggressive any more), and once on two small black bears. I've never used it on Grizzly.
Warning, I digress: It was the middle of the night one winter about ten years ago and I was sleeping. My dog was barking so I got up and looked out the front window. I couldn't see anything, but I heard noise on the front porch, to my right, which I could not see. With my head out the window I asked who was there. No answer. I got my 1911 and went to the front door, turned on the porch light and looked out the window by the door. There were two black bears, about 100 pounds each, rooting through a garbage bag my wife put out there. Relieved it wasn't a bad guy, I got the Counter Assault and, still buck naked, I walked out the door onto the porch figuring they'd run at the sight of my pasty white ass. After a quick glance up at me, they went back to work so, insulted, I sprayed them. They ran like hell and tore through a woven wire fence about 30 meters away, bawling all the way. I never saw them again. That was the result I expected merely from them looking at me. Go figure. Anyway, my son, asleep, two rooms away with all doors and windows closed, woke up coughing. I've gotten some of it in me when the wind was wrong. It's horrible.
Anyway, I've studied bear skulls and the brain housing group is not a very big target, especially head on. Add a bunch of hide and muscle, bouncing up and down in a charge, possible poor lighting and the fear factor, and it may be difficult to make a solid hit with a pistol. The Counter Assault, on the other hand, throws a wide net and it throws it about forty feet out and will stop pretty much anything. Even with forward momentum that might bring the bear too you, I doubt he'll be thinking about much when he gets there, other than getting away. The only draw back is if you get stuck having spray into the wind. You'll get it too. But better that than mauled, I say.