There are too many idiots and drunk/stoned people on live. It gets really annoying.
But, I did have a great time in Halo 3 during a Big Team Battle. I was trying to jack an enemy Warthog and accidently got into the passengers seat. Hearing the other two enemies freak out was great. It went on for half a minute until someone on my team hit it with the laser and killed us all. I haven't played it since. But it was fun.
You know what really annoyes me is people trying to sing over the headset durring a game. That really bad, especially if they can't sing