I tried Combat Arms, TOTAL CRAP. Honestly.
I loved Nexon for when I used to play MS, but since they dared to step in the fragile world of PC FPS gaming, they have given it a bad name. All the hype and advertising makes it look like a somewhat fun game. Laggy lobbies, rude people, unbalanced guns, poor map design, cheesy graphics, and a wacky sniping system made me uninstall this game within an hour of playing it. By the way, it took like 2 hours to download the fucking game, and the downloader used up so much CPU I had to pause the process just to do anything.
Anyways, Battlefield Heroes is amazing. DO NOT say it is a TF2 ripoff after taking 1 look at the cartoony graphics and not reading anything about the actual game. If you have ever played any of the Star Wars Battlefront games and liked it, you will be pleased to know that those games were based on the Battlefield series. Heroes brings back vehicles, control points, and class-based action all for free. My only gripe is that since it just came out in June, it is still very buggy. Plus there's only 4 maps. And 3 vehicles. And 3 classes. And about 5 weapons total. Even worse, they decided to rely on PunkBuster AGAIN for anti-cheat software, which of course gives frequent problems. If you do have problems however you can visit the forums, but good luck contacting EA/DICE. But it's free, so you get what you pay (or don't pay) for.