Simply as that. Nowadays there's no other F2P FPS better than this one. It's essentially like every FPS around, with many gametypes: Deathmatch, Team Deathmatch, King of the Hill, Domination, Kill Confirmation, between others. The big difference in game mechanics is the possibility to use the HRV or Hyper-Reality Visor. This allows the player to look through the walls every few seconds, at the cost of being unable to shoot, and being vulnerable to the enemy. This makes that the game has a much tactical feel than many others, as you can plan your moves and predict the other moves as well.
Another thing is the possibility to customize either your looks and the weapon you're using. From changing your helmet, to changing the weapon's muzzle. All this accompained with a change in the stats of your character, that you should try to adapt to your playstyle. This is great, since you can adapt the weapon to do exactly what you want (using rifles for long-shots for sniping, or adapting them for CQC).
In terms of Graphics they're outstanding. The game look very polished and fancy looking, but there's a downside: the feel that it's not well optimized. If you have a gaming PC, then there's no trouble, but many players will have to play with the minimal settings, as the game requires a lot of graphical power.
There's overall one thing missing in this game, and that is the competitive scene. Even when BCL (a third party website) is constantly making tournaments, the official in-game ranked matches are disabled, and it seems like Zombie-Studios is doing nothing to start with them, even when the "Ranked Match" has been there forever.
All-in-all, if you like FPS, you should try this one. It has a very satisfying reward system , and is absolutely free. Some may think that it's P2W, since the weapons seem expensive at first, but when you have some time playing, you realize that they fit the way you learn to use them. Additionally, they give you a reward pack with free to try weapons when you level up, and there are "Mission Chance Pack" (that you win by doing "missions", i.e. completing some daily tasks) that can give you some perm items, in-game currency or experience tickets.