Combat arms was a great game at the time, but it's so full of hacks now, I suggest paying 20 bucks for Modern Warfare 2, much better. There are almost no hackers, not much bugs and best of all, you can't pay real money to get an edge!
If you can't afford MW2, go for S4 league, it's a great game, with much thought in the anti hacking section and anti bug section, like any other F2P it has some way of making money by giving advantages to paying players, but it's no big deal. Combat arms is fun, but way too many downsides for me.