I have played TF2 for several years now and I haven't gotten sick of it. I think TF2 is really the only game that I can say that about. The mechanics are extremely solid and Valve is constantly pumping new content to shake things up and keep people interested. TF2 should be the textbook example of doing a commercial multiplayer game right. The one caveat may be Steam DRM, but as far the actual game goes, it is head and shoulders above everything else in its field and should become the model for its genre.
I've been playing TF2 since beta and I'm still learning how to play! It's a very easy game to get into but it's also a very deep and rich game that rewards continued learning. There are also many, many different styles of play.
http://steamcommunity.com/groups/newsyc