I've been playing xbox games for about 2 years now, I haven't really found a game that I got truly involved in. I played Halo 3 for a while and it held my attention pretty well, but I just couldn't get into the multiplayer portion of the game. To me, the multiplayer(online) side of a game is what really decides how long a game will stay popular. If the mutliplayer sucks, the novelty of the game wears off with the completion of the single player campaign. Call of Duty 5 is probably one of the best multiplayer games I've come in contact with for the Xbox 360. Wait times for online games are minimal, gameplay isn't too unrealistic and the controls are extremely easy to get used to even for new players. The variety of maps is decent, and after playing a few games you quickly find your favorites. It isn't as good as it could be(friendly fire would make more sense, and destructable environments similar to "Battlefield: Bad company" would put the icing on the cake. Levels aren't difficult to attain, so frustration doesn't set in like other online multiplayer games, and the perks and weaponry upgrades for earned levels are fantastic.
I haven't been into shooter like this since Counterstrike. I highly recommend it.