One of the best tactical shooters ever made, not to mention free.
The maps are well designed and fun to play. My personal favorite is Pipeline, which has a mix of outdoor and indoor terrain, plus distant firefights and close-quarters combat. While the map Bridge Crossing seems small, you will have a fun and long-lasting time trying to make it to the other team's side of the bridge. The new map Steamroller (available on versions 2.7 and 2.8) is massive, featuring 6 stories you can explore. The co-op maps featured on those versions of the game are also exciting. The AI enemies you play against in these maps aren't half bad and can sometimes prove to be a challenge to defeat.
The overall gameplay is highly realistic and difficult, but a fun experience if you take the time to get good at the game. When you first start playing the game you will probably be killed a lot, but once you learn the pace of the game and excellent tactics, you'll be racking up kills. While the gameplay can be very slow paced, its still very good. The game can get suspenseful as your teamates die and you become the last member of the team alive, sitting in complete silence. Or it can be exciting and feel like a real battle with players shouting out orders and reports over the radio and people dying everywhere. The excitement builds when you're alone and you run into an enemy and grenades and bullets begin flying. One of my favorite parts of the game is the rag-doll effect when players die. A dead body will almost never look the same.
In short, this game is a fun and exciting experience that is well worth the free download. The army did an excellent job with this game.