For the thirteen bucks I got this for, you simply can't go wrong.
The game's been out for nearly eight years, and some people who you might run into online have been playing all this time, or close to. If you're a starting player, which we all were at some point, this is bad news for you; some players have dead-on accuracy, know every map's layout like the back of their hand, and have a (figurative) book of tricks to help get the job done faster and more proficiently. It might take a while before you can keep up with some of your fellow players, but once you get good enough, it feels totally worth it.
Graphics-wise, the game came out in 2000, what can I say. It definitely gets the job done; no flair or anything special, but still decent. The sound effects are great, with every gun sounding just as powerful as it should. The voices are also a strong point; although somewhat cheesy-sounding, try not to smile whenever you hear the announcer shout "HEADSHOT!" Absolutely classic.
The real heart of the online game, in my opinion, lies within the mod community. There are tons of talented gamers who take it to the next level by creating their own maps, game modes, player models, weapons, sound effects, etc., thus completely changing the way the game is played. I've played games in a Simpsons-themed world (complete with sounds from the show), a game where every player was modeled to look like Master Chief, and, my favorite of all, a zombies game; terrorists were headcrab zombies from Half-Life, and the CT were humans barricaded inside of a decomposing mansion. I think the rest is self-explainable. This is great if you ever get tired of the true game mode; hell, if you look hard enough, you can even find basic run-and-gun deathmatch games.
If you buy the Anthology like I did, you'll also get Condition Zero, which is more or less just CS with bots and objectives (kill a certain amount of enemies, complete your objectives in a set amount of time, etc.). It's passable, but nothing great. Also, I'm not sure if I was alone in this, but after I installed the Anthology and downloaded Steam, I was surprised to find Deathmatch Classic and Day of Defeat, along with one or two others, in my Steam game library. I'm not sure if these come free with Steam, or if they're in the Anthology or what, but they're great, especially Deathmatch Classic; it is the definition of mindless fun (I'm tempted to say "so bad it's good", but I won't).
Overall, the Counter-Strike Anthology is a great choice, especially for the price; I found my copy at a Best Buy for $12.99. Not only do you get one of the best PC shooters out there, which by itself is worth the thirteen bucks, but you also get its single-player counterpart and the bonus games that you get after you download Steam. Best of all, pretty much any PC can run it (if you can't, I think it might be time for you to upgrade). I would recommend this to anyone who's willing to put the time into it; if you are, you're sure to have a great time.