Just realised that I haven't posted in a long while. Well Halo3 has been and gone and now I'm getting into Call of Duty 4:Modern Warfare. The game play in CoD4 is much better in my opinion than in H3 - it is also a lot more fun. The single player is quite short but there are 4 ways of playing it, Recruit, Regular, Hardened and Veteran. So once you have played a level through you can go back and change the class. In multiplayer various game types are opened up as you progress and gain experience points. Also different weaponry is available, along with perks, as you progress.
It is one great game - let's do this :)