I finished the campaign yesterday afternoon. I'm not going to give any spoilers away, but while I like the new additions (new enemies, new weapons, better AI, and good backstory), I didn't feel like there was any big task at the end of the game. I just finished a level, and then the credits started rolling.
Multiplayer - which is the real reason I wanted to play - seems to be working fairly well, though I have a few comments:
1) The wait to find ranked game is really long at times, and there's no indication that the system isn't just hung. This is bad. I'm not planning on playing a ton of ranked games as I do it more for pleasure, but I do want to get a decent ranking so that I can match with my friends when we do want to rank. I think I'm at 6 multiplayer and 4 single-player right now.
2) The maps are pretty good, with good variety in size and shape.
3) The banshee is pretty much eviscerated as a dominant weapon. That may be a good thing - it's certainly true that Banshee whoring can get out of control on Halo PC, but I'm finding it much harder to do anything useful in the thing.