Perhaps the best FPS game of all-time, though of course with some notable but notwithstanding flaws.
The pros: amazing graphics, even on my relatively crappy PC; smooth motions with a good connection, sharp textures, incredible lighting and sound qualities, imposing AI, an impressive array of weapons, and a cool buying system for equipment.
The cons: imposing AI just too damn imposing--even on 'easy' difficulty they still get miraculous headshots with shotties from 100 yards away; they can somehow discern that, from that distance, you have a sniper rifle before you even shoot and whence you begin to aim they begin to weave; they often withstand a couple .50 caliber rounds to the face without death; the only change in difficulty is that they jump more frequently and move with more intelligence. Basically, that is its only flaw IMO--then bots' rationality.
Other than that, especially for online play, CSS is nearly immaculate.