I played a bit online last night. However, it didn't seem like I was connecting properly most of the time.
The 2 or 3 times I got into a big Team Deathmatch game, it was pretty fun. The controls take getting used to. Everyone seemed to be exploring more than playing. Still, it was fun with rocket launchers going off everywhere. I hopped in an ambulance and starting running people over. If I got one, I'd turn the sirens on for a second. I also climbed a 50ft billboard with a rocketlauncher and had fun blowing up people and stuff from way up there.
I didn't get to try any other modes. Seems like there's 8 or so different games, most of which have an additional team option, too.