While I'd tout this as one of the best games of the year -- the singly-player campaign is absolutely fantastic, near-perfect really -- I have some personal qualms with multiplayer.
I give Call of Duty 4 kudos for innovating a new experience system for their multiplayer, and it works beautifully. I expect to see their efforts mimicked by other studios in the future, especially by the likes of Ubisoft with their Tom Clancy franchises. However, in contrast to Team Fortress 2, CoD4's multiplayer levels are sometimes awkward and inconsistent. Whereas in TF2, players can really enjoy any one of the many classes present in the game and play it to enjoyable effect, the maps present in CoD4 generally tend to favour only one or another style of gameplay. For example, some levels are much better suited for sniping and others for in-your-face close-quarters destruction. But the level of grey between extremes is not as balanced as in other multiplayer games I've enjoyed.
And not to bitch too much, overall this is still an excellent game. If you're looking for some light family fun, this ain't it. But if you're a true blue shooter and a fan of Rainbow Six, this will be right up your alley.