When I was 9 I got to see my mother in a hospital after she flew through a windshield. The driver fell asleep and hit a guard rail (cut the engine in half and exited through the roof of the car). She wasn't wearing her seatbelt and went head first through the windshield. I will ALWAYS wear my seatbelt, and so will everyone else riding in my car.
I pretty much always do, but I'm not totally convinced that they are always effective. When I was young(16-4 months after I got my liscense) I wrecked my dad's '78 Civic CVCC. The police said it appeared to them that the car rolled end over end at least once and sideways 2-3 or more times. I was NOT wearing my seatbelt and the driver's window was down. I walked away with out a scratch. When we were cleaning out the car at the wrecker yard the next morning, I tried to sit int the seat and the only way I could do that was to scoot way down in the seat and lean about halfway to the passenger seat due to the caved in roof. That car had shoulder type belts and, had I been wearing them, I would have been killed or permanently paralyzed. In fact, when I got back to the scene of the accident after going for help, the officer didn't believe I was the driver because I was still alive. As I said though, I do wear my seatbelts pretty much all the time; it's the law in Indiana.
Horsepower is how hard you hit the wall, torque is how much of the wall you take with you.
I do wear mine 99.9 % of the time that I'm in a vehicle. I have had a similar experience as diamondlarry though, where the 1 time in 1000 I didn't have mine on I had a crash and ended up with mostly scratches. If I'd have been in the car, I may have been decapitated - T-Top car and it flipped and slid on it's roof. And it crushed the driver's side A pillar. God was watching out for me that day I assure you.
So, while I believe that they have benifits, they can be detrimital in some cases. It's the odds that dictate why there are seatbelt laws and such.