You can still play Halo and Halo 2 on the xbox 360 (No X-Link Kai though ;_;)
Halo 3 is only worth owning if you have XBL.
The single player if fun, and co-operative is good too but the multi-player is where the awesome is at.
"I've heard it's just a bunch of little kids playing anyways."
Whoever you heard that from, they must have something wrong with them to complain about that.
Yes there are quite a few little kids playing but most of them are noobs, noobs are fun to pwn and there's always the mute option if they have team speak. I know this little kid who is quite skilled, a legit general.
You really have to experience it for yourself.
If you have a friend with a 360, XBL, Halo 3 and a spare controller, just ask him/her to load you in as a guest one day and play some social. They'll know what you're talking about. You can make up your mind then, or just spend every weekend at your friends house, nightmarishly addicted to the Halo 3 online multi-player experience.