Annoying game that looks great.
Above all it violates three rules of modern games, four if you add having fun. First, you are only able to save at save points. Second, enemies continuously respawn unless you kill just the right combination. There is rarely any indication what that combination actually is. Third, you cannot reduce the difficulty level mid-game. This is important to retain the fun factor if you get stuck on a level.
I am genuinely baffled as to why the designers chose to violate these rules; computer games have been following them successfully for years. Given the non-skippable cut scenes, especially for the opening credits, I think they were a bit full of themselves.
Reviewers have praised the AI for your companions. I don't know what their talking about--it's horrible. I can't count the times I've died because my allies got in my way, did something stupid or simply didn't help at all. (The funniest moments, though quite annoying, were when I bounced grenades off their heads because they ran in front of me.)
I was hoping for a FPS with tactical elements; all too often I found it was just a frag fest and I had to just rush in, all guns blazing. (Ironically, more than once this ended a sequence almost instantly--it was truly bizarre.)
Call of Duty 4 is one of the most disappointing games I've ever played. While playing, I often shook my head at the lost opportunity. In many ways it was better than Call of Duty 1 and 2--it didn't feel quite as scripted--but it lacked the fun factor, especially of Call of Duty 1. Needless to say, I'll be auctioning this game on eBay as soon as my kids finish playing it.