Campaign should take no longer than 8 hours to get through. I did the first two this morning in about 40 minutes. But it's getting together with the multiplayer with friends that makes it all worth it. And the addition of Forge is what is going to make this the best Halo of the 3.
Bioshock was the ****, too. It does have replayability, though. Just set it aside for a couple months and you'll be eager to get back into it again. I know I'm going to.
And jth...we get it. You don't like Halo. Go wiggle around your Wii remote (which bored me to tears so I sold it without even thinking twice) or put in whatever junk has been thrown out for the ps3. It's obvious you haven't gotten into an evening of some Halo online with a group of friends. It's a blast, especially if you don't take it as seriously as the little bastard kids do.
Before you say I don't have a social life and that I don't get outdoors, I'll let you know that I'm happily married with 3 pets and a house, I work full-time, mow my own yard and am training for my 2nd marathon this December.