The game is what Doom 3 should've been.
You are a guy who was born while the world was still as we know it, and then destruction came. Apparently, the monsters you see are somehow connected to us, and there are some of them with mind powers, and you are the only person who is immune to them. You make a promise early on in the game, and the rest of the game consists of fulfilling that promise, while traveling the world of metro, and even the surface. Story so far is strong, with a great sense of mistery and character.
Gameplay wise, think of this as a Bioshock meets Cod like FPS, you have more than one way to approach a situation, stealth/action. The A.I is pretty good, from the human side, and rampant from the monster side. The game IS very much a scripted affair like COD, but where COD goes for the bang, this game goes for the atmosphere, going through highly detailed subway tunnels with a NPC buddy using his flashlight in a play of lightning games that will make you wet for the visual effect that the world around you achieves.
In between these dangerous travels you have the subway stations that are markets, or small towns if you will. Ammo is the currency you use to buy weapons, medkits, and filters for your mask, which you will need in the surface and in toxic areas underground.
It's not a revolutionary game, but it really is a very very good looking FPS, that has personality, and will keep you glued to the screen. If you can, get this game.