When it first came out it was the best FPS on the market. And maybe still is.
It's used in PGL and there are a big load of championships with it.
CS:S ("update" for CS 1.6) is fast-paced. In every server you can have different weapons and gameplay. Servers can add mods which can't be added in BF:BC2, MW2, or other new FPS games. What I mean is custom maps. It's hard to download all the current maps found for CS:S, actually impossible, unless you have a ~1TB HDD, a loving fast internet speed and a big load of freetime.
Plus, every round you can choose different weaponry, there are no classes, there is no medic kits, which actually makes it special, you need to fight to survive, not to go kamikazee and hope a teammate magically revives you or you receive your health back.
No man, if you lose a bit of health you are with that health for the current round.
Plus, there is a mod called "respawn" which makes you respawn once you died, making it more distractive.
That list can go on and on, but I can't be arsed to continue.
Source: First time I played CSS 1.6 when I was ~5 years old, in LAN at a internet cafe.