I played the 360 multiplayer beta, and damn was the multiplayer fun.
I really don't like the deathmatch style of halo 3, everyone jumping around and stuff. I'll defiantly be picking up cod4... im just not sure for which system. I have both ps3 and a 360. Guess I'll wait for reviews!
The PC demo is amazing - the graphics are alot better in the single player than they were in multiplayer. Not saying multiplayer was bad, it just seemed to be missing some of the lighting effects.
If anyone is curious, I get about 30fps at 1280x720 with most settings all set to the max except vsync on, soft shadow edges and antialiasing off, using my 20" iMac 2.4ghz (the new ones). Not amazing FPS, but its very playable, and looks gorgeous!