Some of the most innovative, unique, and addicting gameplay ever. Instant classic!
There is only one thought, likely synonymous with every other gamer out there, that I can voice: this is the second greatest multiplayer experience ever (just behind Team Fortress 2 and just above Left 4 Dead; incidentally, all three are by Valve). I mean, there's just a satisfaction of knowing that when you kill an enemy, they stay down, motherf****r.
The strategy and teamwork that's required to win is fantastic - you're not going to get anywhere without teammates, just like real life. The realism is palpable - there are still a lot of cartoony aspects, but they're not particularly prevalant. Its great to know that all across the map, individual firefights are happening when you've got a large number of players. When its just 4v4, its even better - a single kill or death could tip the balance of the game.
And then there's the buying system, by far the greatest thing that Valve has cooked up for this all-time classic. I mean, CS:S would be great even without this, but they just went above and beyond here - having to start out with just a few hundred (or thousand) dollars to buy all of your equipment, and then struggling to gain more cash to buy the better weapons as the blitzkrieg-fast rounds fly by in spasms of super-intensive combat and strategy.
Just be warned: the majority of players that are playing by now, roughly 5 years after release (which is still a staggering amount) are pretty much all hardcore, really well-skilled players. You WILL BE KILLED, many times over, until you can adapt appropriately. Beyond that, this game is amazing.