MODs are where it's at...
Graphically, the Source engine is still beautiful, despite being a year and a half / two years old. Character models are glorious and detailed and move around realistically. The physics are great and it's definately satisfying to cap some guy in the head and see him tumbling head over heels over a ledge. The weapons are modeled after real world weapons and look and sound convincing, though I must admit I've never personally seen or heard many of these up close so it's hard for me to attest to their accuracy.
Overall, CounterStrike is a good all around game. It may be hard to get into it at first, and the average skill of your opponents may be overwhelming, but it won't take long for you to jump right in there and start chalkin' up some kills