Modern Warfare: Yes, it's pretty damn good.
Modern Warfare 2: Depends, if you're coming at this from an Xbox live perspective it will be fine. If you're used to playing the Steam, Battlefield, Unreal, Quake or similar PC centric shooters you are going to find it a shock. Not least it's iffy definition of acceptable ping, but it's also vey, very restraining and limited in it's scope. It's slick, just about 10years behind what you would consider the minimum for an online shooter to do (except Special forces, which is good).