I just can't get myself to enjoy a FPS without mouse control (unless it's a pre-mouselook design, like Disrupter), so I'll talk about Halo 2 after it comes out for the PC.
But I ended up hating Halo 1 due to the repetitive level design. The game did combat very well, but the game got to the point of recycling the same rooms repeatedly (made even more tedious by the inclusion of arrows on the floor, pointing out that level design was so repetitive that they had to provide visual indicators to ensure you didn't go backwards), and the 2nd half of the game even had you going backwards through many of the earlier levels, all the way back to the starting point.
But I still love properly done FPS games.
Far Cry is by far my favorite of all time.
Doom is still amazing (it really helps to use Jdoom so it controls and looks like a modern game)
Call of Duty 2 was very intense
Tribes 2 is still the best team-FPS I've played
and the non-traditional FPS games like Operation Flashpoint and Deus Ex are really fun. (not that those two are anything like eachother)
I don't really like deathmatch though. I'll play games like Quake 3 / Unreal Tournament 1/2k3/2k4 once in a while, but it doesn't really manage to hold my interest, and usually gets uninstalled after a few days. Every 6 months or so I try to "get into it" again, and the cycle repeats.