Played a bit of online multiplayer last night. Had a blast with some buddies. It's already better than Halo 2 (which was a disappointment) and I haven't even tried campaign yet. It might be good enough for me to buy a 360!
You might want to check out the Forge map editor. People online are already abusing it (spawning tanks next to themselves while giving their opponents Plasma Pistols). However, if you're just playing with friends, its great. Image running through a desert battlefield, and out of nowhere a Scorpion Tank comes flying at you from out of the sky. Its can be pretty funny at times.