My friends and I have been having a blast playing Team Training mode (the only matchmaking mode for Team Slayer w/ guests) on xbox live.
Playing online is fun, but playing with 5 friends the same room on two screens is amazing. Team gaming takes on a whole new nature when you are playing w/ your friends against a team of strangers.
It's amazing, with the life extension Xbox Live provides games, $50 doesn't seem like a bad price tag for a title. Usually even the best titles get shelved within a month or two. Halo2 is still being played strong @ my house thanks to Live. There is just no comparison to human opponents; the AI of most titles grow boring and repetitive, with poeple the game continues to remain dynamic. I'm always learning from my opponents, good and bad.