If you don't mind the artstyle or shooters in general, this is one of the best free to play games out today. After 1000s of hours committed into this game, I can safely say that the 9 distinct classes will surely become much more than just a random soldier in most other shooters. There differences are so apparent the first time you take the handle of them. From the pyro's flamethrower to the sneaky spies cloaking device, you know you fell onto something wacky as soon as you picked your class.
Of course, this may throw off some gamers who are used to their gun of choice, but its the uniqueness of the classes that will make this a great experience. Each one plays differntly, and the unlocks that have been put into the game (and are still being put in due to updates) only makes their experiences even more fresh. This does make for some balancing issues at times which is the .5 score drop, but overall Valve tries its best to make everything fun and clean for the average player.
The best part (at least for me) is that there is no real such thing as an instant kill weapon! Its all dependent on that little health symbol on the corner which can drop quickly, go up quickly due to healing and so on. There is no real chance of cheap deaths in this game (well there are critical hits...) and the fighting for many classes up front doesn't deal with any last second lag or whoever fired first wins mechanics as in other games.
Overall its a very polished game. Everyone should give it a try! It is free after all :).