Check the dates of the reviews. When the game came out there were some serious performance issues. They've made a lot of progress on most fronts though and the game should certainly be playable at least on most reasonably up to date machines.
One problem which has still been lingering though is that the game is very much CPU bound. Around the same time that the game came out I picked up a brand new, high end video card. I still get graphic lag though when something big happens (for example, and enemy coming onto the screen and a fight ensuing - d'oh!). This happens despite the high end video card, screaming monitor, and ping times of < 30 ms, so I think that it really does have to do with the widely advertised CPU issues.
My CPU is a few years old, but to replace it I'd have to replace my motherboard as well and it would require me re-registering Windows, etc. and is just not worth the hassle for me right now - especially since it's still a pretty good CPU and a game like CoD:BO should run on it fine. While the game is now playable and I'm competitive, I'm still holding out hope that a soon to be released patch will clear things up completely for me.