I think Halo 3 will be a good game- a regardless of whether it's a good game or not I can guarantee that alot of people will buy it (mainly hardcore fans of XBox 360 and the Halo series).
The main reason I'd buy it is because I still enjoy playing Halo, and Halo 2 for me is still a treat. I know alot of people will disagree with me on this (everyone is entitled to opinion) but I wouldn't care if the graphics weren't much improved, ad only a few more weapons were added (or taken away). It's still Halo to me, I wouldn't want a different gamestyle or different engine for a sequel to a game, and this may very be the best choice (Tomb Raider's gameplay has seldom changed, but it's still going strong)
Of course, I don't know much about the actual game, so I'm up for debate on anything I've just said.