XIII offers you some things that other FPS don't give you. XIII is very underrated, and brings a great FPS experience.
The graphics in the game are cell-shaded, like I said before. It really cool to see a game with cell-shaded graphics. Its new and different for the FPS genre. Also since the game is based off of the french graphic novel. So the developers decide to stick with the graphic novel look, and make the game feel more like a comic book. The comic book feel also comes into the gameplay, but I'll discuss that later.
Gameplay is a bit different than most FPS in that you can pick up certain items like chairs trays, and even glass shards. Thats right, you can break windows and use the shards to take out your foes. The comic book feel comes into the gameplay by showing if an enemy came through a door or if and certain objective is near. Also, one of the best things ever, if you snipe someone in the head it will show you where at in the head you sniped at. You can never get tired of that.
The sound quality in the game is pretty cool. You can easily tell who is having a conversation with someone, because a voice balloon comes in and shows who's talking. And there is someone literally singing in the game. It doesn't mean a thing to the game, but I thought it was funny. The voice acting is really great with a great cast like David Duchovny, Adam West, and Eve.
Overall, XIII is a great innovative game. The bad thing is that no one really played it. If you are a big FPS fan, you should give it a try.