CSS (Counter-Strike Source) is basically a rehash of the original, but with much better graphics and the addition of dynamic physics.
GRAPHICS: 9/10
Not very compatible with some nVidia hardware or drivers, but ATI owners will get the best out of this. Everything according to looks is fantastic, and it runs surprisingly well on even the low-end pc's.
SOUND: 8/10
While some of the weapons still use the sound effects from the original, the new ones are a big improvement. Body effects are disturbingly real; you can hear the *crack* of the skull when you land a headshot or the realistic sound of a knife tearing into flesh and ripping its way out. The re-used sound effects will bring this score down, but not very much.
GAMEPLAY: 9/10
While the only new thing CSS offers gameplay wise are the physics, it doesn't bring the score down at all. What could possibly be better than a legendary classic putting the latest technology to good use? One thing I didn't like is the number of bugs. Very rarely, the game will just crash into the desktop, and the physics bugs (I.E. barrels knock you back when you bump into them) are annoying, but I'm pretty sure Valve is working to fix these right now.
CONCLUSION: Though you won't find a lot of new content, CSS is still worth getting into.