Just finished SP. It was one of the best single player experiences I have had in years. It was LONG (unlike most FPS nowadays), but really got you into the action. The storyline was solid, the action was great, and the replayability is huge.
As for performance, I have an Athlon X2 4400+, 2GB RAM, and a 7900GTX. When it first selected medium, the performance was REALLY crappy. However, I turned a few settings down to low and my FPS improved by 20 or so. And all stalling/freezing between scenes and what not was eliminated. The settings I turned to Low were Sound Quality (which actually was a problem.. I was unable to tell when helicopers were REALLY close or just somewhat close. And I couldn't tell which direction they were coming from. But I attribute this to the fact that I am still using my onboard sound..), Volumetric Effects, and Shadow Quality.