Quality multiplayer that doesn't try too hard.
The myraid of servers in the game usually have balanced teams, and many maps with motivating gametypes make this very well rounded.
All I can say is that if you are only interested in realistic gunplay then you will not enjoy this game. Everything is very silly and animated, but that doesn't mean this game isn't for hardcore players.
This game takes a lot of skill to master but its still easy for a noob to get some kills.
Some classes, (like the engineer) don't require you to even aim to get kills. On the other end of the spectrum, the sniper must be accurate in order to get his kills.
I don't really have much else to say, this game succeeds at balancing every class, and I recommend it to anyone who enjoys multiplayer. I would recommend the Orange Box, but to be honest I am not that big of a fan of Half-life 2 (see my review). However, Portal and The Original Half-Life will make it worth your money.