Halo 3 is actually pretty good. Much like all the FPS games that have come out on the PC in the past like Quake and Unreal. I still think Half Life is the best single player experience for an FPS.
While I enjoy playing FPS games a bit more on the PC (I think the controls on the PC are far and away better than what you find in a console), the latest FPS games are pretty slick. Rainbow Six Vegas is a fun game on the 360. Halo 3 is a very good game. I don't play it online, but it is fun split screen with my friends. Oh, and they look fantastic when played on a 42" widescreen.
All that being said, in my opinion, sports games, i.e. football and basketball, are much better on a console than a PC.