Wait'll you play COD... I loved HALO with the kids, then they got me playing people online... Fun times, and the goofy weapons make the game unique. Then though, we got COD4... I played it against the kids, before I even played the campaign actually, and had fun, but then they got me into playing online... It propelled me into abandoning one game I was in the midst of to play the COD4 campaign just so I got better at playing COD4 online.
I'm not much of an online gamer, but that became some addicting ****. I'd play till 4 a.m. some weekends... It became depressing almost. Random grenading, runnin' and gunnin', mowing down kneecaps, upgrading the M4 (my absolute favorite weapon)... COD4 is just fun times x10.
HALO is still fun too, don't get me wrong. It's just different fun. Both are very, very, very fun online games... Trust me. You'll dig both I think.
I'm a cheap bastard too, so if you're worried about cost the gold membership is cheap. Currently our Live is down and I went through withdrawals about 3 months ago when we lost it... Problems with the router. It's something though that I really look forward to getting into again. COD5 is supposed to be fun but I'm thinking the limited upgradeability of the WW2 arms will be a letdown to the gameplay. I mean you can't add grenade launchers to every gun, laser scopes, etc. COD4 probably will be my go-to when I get back online.