Aiming feels a lot better than GTA4. You have much better control of the sensitivity, even though some users would probably want to be able to turn it up even higher. I think I'm near max, which is not nearly as sensitive as other shooters. I'm at a sweet spot, near max, which might be 2/3 of what other shooters may offer.
As for the other shooting intangibles, like sound and impact, this is top notch. Shooting a guy off his horse, and watching him very realistically fall to the ground, where his own horse tramples him, is pretty much video game magic. They did a really nice job in making all of the ragdoll and tumbling look realistic, rather than a human meat sack without any bones in their body, like many games do. You can shoot a guys feet out from under him while he's running, shoot off his hat, shoot a gun out of his hand etc, and all of the reactions look very convincing. I'd say real, but I've never actually shot at anyone before. Cover mechanics are simple. It would be nice to have some more cover-to-cover type features, but what's there works. If you're in cover underneath a window, and hit LT to aim, your guy will smash the window with his elbow before popping up, which is a nice touch. Oh, and the guns don't produce bright muzzle flashes, or shoot out huge flames like in an action movie. You just get a nice powerful pop and some nicely done muzzle smoke, like you'd see in a western movie.
The one weapon oddity I noticed in multi-player, and not sure about single-player, is the throwing knives. Fun to use, easy to aim, and the knives stick out of the bodies and the enemies react according to where they are hit. But for some reason if you throw a knife at a wall, you don't see it sticking out of the wall. Thought that was kind of a crappy omission. It would be cool in multi-player to hear a knife whiz by, and turn around to see it sticking out of the wall beside you.
If you want to see a really cool combination of physics and animation, find a steep hill, take a short sprint, jump and watch your guy roll to the bottom. I think I had a 20 second roll where I stopped when I hit a tree. The guy does a bit of a tuck, when he can, and goes tumbling in a comical manner.