I just picked up my copy on Friday. I was a huge Halo fan on the original xbox, and Halo2 was "just plain badass" when it hit stores. I played it nonstop for months, it seemed. Halo3, honestly, is a disappointment. It's not the huge improvement over Halo2, that Halo2 was over Halo. I have a 46" HD TV, and the graphics are certainly an improvement over Halo2, but nothing stands out as "wow, that looks amazing!" Except for the reload button, the controls are the same as they've been since Halo. They've added a 3rd type of grenades (insert sarcastic "yay" here), added some cooler vehicles and weapons (which IS a long awaited addition). The campaign is way too short, as it only took me one day to complete it. The multiplayer is a bit better, with much better maps than halo2. They added the ability to record your games now so you can go back and watch them (not really sure why you would want to sit and WATCH yourself play).
All in all, it's a good game. If you're a halo fan, you'll like it, and find that you HAVE to play it. But after I played through it....the multiplayer isn't a big deal for me, personally. One thing that is very cool is the fact that you can now have 3 other people across xbox live join you and all 4 take on the campaign mode.
BUT...I got spoiled. I bought Rainbow Six Vegas back in November when it first came out, and that game is still leaps and bounds above any first person shooter I've ever played. I keep trying to compare Halo3 to it (even though I know there were much different intentions with each), and Halo3 doesn't compare. But....for what Halo3 is supposed to be...it's a great game.