The reviews for it are lower than MW2. Probably reviewers are starting to understand that it's not that fun to play the same game over and over again with new paint.
The only game company that makes unique, purely enjoyable experiences anymore when it comes to FPS, is Valve, the guys who make Half-Life, Portal, etc. They are the only ones in the business that don't do rehased military shooters that we've played countless times, or yet another Halo that's the exact same game it was the previous.
Oh, and for those of you who have played the original half life or have wanted to but it's too dated, the remake with new graphics has been going on for a long time now. Looks awesome.