Well I've just gotten Halo 3, borrowed it from a friend.
This game is a bit over rated, I'm not saying Halo as a series is bad, but this game just doesn't have the same feel it used to, and for some reason, Halo 2 seems a little better. Sure the graphics are splendid, it's Halo friggin' 3, and the multiplayer rocks. But this is not the end all of shooters, honestly, Call Of Duty 4 will outlast it.
I have to say, after playing Call Of Duty 2 for so long, I felt like an outsider playing this game. The controls are compeletly different from any shooter out there. It took me forever to tune my mind into pressing RB instead of X to reload, which usually screwed me over in the beginning. Plus, scoping, which is usually reserved for the left trigger in any other shooter, is back to the thumbstick, which really took the sniper out of me. What I do love about the controls is that you can reload your dual wielded weapons independently with the bumbers instead of at the same time. Overall though, once you get to used to the controls you will feel right at home. I was also confused for awhile because I though they had removed the flashlight function, since it's not mapped out in the instruction book, but I later found that any direction on the D-PAD toggled it.
There's a lot more added to Halo 3 this time around. The equipment mechanic feels cool at first, but after a couple of hours I thought of it more as a gimmick. However, I immediately fell in love with the new vehicles and weapons. The Forge and video editing system are pretty cool too.
In presentation, this game just blew me away. Some people say that Gears looks better, but I say that even if Gears has more sparkle, the varying color pallette and environments makes Halo 3 much better to look at. Plus, the blurring animation on most shooters for when you turn around is tuned down in Halo, so it was easier to keep track of baddies. The water and environment interaction effects were pretty nice as well. Water moves realisticly when you wade through it, and leaves and boxes move around as you blast through them. But, while the environments and everything look nice, I think that the character models, specificly your fellow marines, could use a little work.
The sound design is the real selling point. It really sets a mood. The game can also tell from how far away a sound is coming, so it is muted accordingly. Explosions, gun fire, etc, sound perfect too.
I haven't gotten to play the multiplayer yet, but you can ask anyone and they'll tell you that it rocks.
The game felt a little short, but the free form way that you can play through the nine levels adds some replay value, as well as going back to film some kickass stunts and what not.
Overall, buy this friggin' game.