I beat it today...
TBH, I didn't have high hopes getting into the game. I wasn't even going to get it until I watched the Forward Until Dawn videos that got me pumped for the game. Others have said what my own thoughts are... but I'll recap ;-) .
The opening scene is INCREDIBLE. Those graphics blew me away, I couldn't even realize it was CG for a few minutes, they were that good.
Story-wise, it's a lot deeper. It has multiple tangents, with Cortana's rampancy subplot, the politics involving the USC Infinity, the main enemy, etc. There were things let up for explanation, especially concerning the scene with the Librarian, but that may be by design.
Graphics definitely improved, and I liked the new weapons. I hate those flying things though, drove me nuts!
The soundtrack was fantastic, I bought it soon after finishing the game. I loved the new gun sounds... everything sounded more realistic.
The ending was poignant and powerful. The multiplayer has been great so far.
Halo 2 has always been my favorite game of the series (Halo: Reach was awesome as well). I thought Halo 3 was a sore disappointment, to me, this was a breath of fresh air.