I've played americas army, its damn difficult. I love the fact that you have to train and pass exams etc. to qualify for certain maps and upgrades but the actual gameplay closer to counter strike but not quite as good. Its a good piece of propaganda for the us military though, really make you want to sign up.
Afther you download it you will need to get the latest patches and the latest version of punkbuster, put them in your download cue from now.