It's tough for me to say. I got really excited for Halo 3, played it and enjoyed it, but then I suddenly got really, really sick of it. Like the kind of sick of it where I never wanted to play another Halo game ever again. I think it was the gameplay that bothered me, and obviously, that's kind of a big thing that get sick of.
But a few days ago I borrowed ODST from a friend, and while the plot was shit and the characters were incredibly unlikable, I thought the gunplay was actually really good, and it was nice to have a split screen game again.
Now for Reach, I think some of the stuff that they're introducing (i.e. Forge World) is awesome. If the story isn't terrible, I think there's a chance I'll get it, which I'm very surprised to hear myself say.