Very easy to get hooked
Thinking about previous shooter of its kind (Unreal Turnament, Half Life and so many more) I can only credit the developers for a job well done.
Ever growing variey of maps, with intensive activity, realistic environment, body movements close to perfection offers users with strong enough computer, display adapter and broadband connection, the ultimate experience. Add to that the fact that it is free for all and get the ultimate package.
I strongly suggest downloading the 2.5.0 version, go over the basic training (ingantry, medic, sniper) take the time to take special forces training and enjoy excellent maps exclusive for that class.