To date, the only 10 I've given out, but when a game like this is as perfect as it can be, who am I to deny it?
There are 9 playable classes, each with an unlisted but quickly realized set of distinct advantages and disadvantages: The Heavy is strong and has a lot of health, but is slow and easily killed with a headshot from a sniper for example. Each class needs to be played to its strengths, and avoiding their weaknesses in order to maximize your effectiveness. The insane level of balance means that there will never be a team of all one or two classes, as the team will often suffer greatly for that. It includes its own talking system so its easy to strategize on the fly if you'd like to hook up a mic, or don't and keep an eye out yourself for what's going on. I found the only real need for using the voice communication is to tell your team if you see a spy, who can disguise themselves as a member of your team and kill you with a backstab if you're caught off guard.
The modes vary from Arena which is basically a "last man standing" game with no respawns, to a capture the flag, to a capture the point game, to one of my favorites: payload. Payload puts one team on defense and one on offense, as the offensive team tries to escort a bomb from their starting area to the ending located deep in enemy territory.
Team Fortress 2 offers a lot to people who love FPS games, and to those who just love team-based strategy games. The point of TF2 almost never boils down to "who's the better shot" and is often reduced to timing your attacks and knowing who to kill first. The strategy involved will initially throw you off, but the fast-paced gameplay makes it quick to catch on and learn.
TF2 is everything it needs to be to be amazing: fun, fast, balanced, and it offers easy controls for new players as well as it offers skilled players the opportunity to show it off. I've been playing it constantly since I bought the Orange Box, and I've yet to find a single flaw