Just finished it. Good, but not great. Certainly not as good as all the hoo-hah, but at least it wasn't nearly as overhyped as Halo 2. I agree with Swantz in that it felt a little cobbled together, and way too many things were left either unexplained or completely ignored in favor of more explosions.
Anyway, it was nice that they've unabashedly set up a further sequel without making any obligations. It's a great universe Bungie has created here, and hopefully future games will be able to explore it a heck of a lot better than Halo 3 has.
Edit: Did anyone get a similar feeling that the game lacked the epic feel of the previous titles? I mean, there were plenty of "wow" moments, but there simply wasn't any grand, sweeping moments that really knocked you back onto your seat with a startled expression on your face. Seriously, I expected a hell of a lot more from the campaign of what is supposedly the reason incarnate to buy a 360.
Good? **** yeah. Killer app? Nope.