It's a good game, but it's relatively short (not sure about the exact time since I juggled it between 3 other games) and there is no real reason to play through it more than once since there are no unlockables that I know of.
The controls on the Xbox version takes a little while to get used to. It uses the default Halo config with a few tweaks:
L trigger zooms in.
Digital pad left/right leans your character. (remember than Bungie?)
Digital pad down for flashlight.
Digital pad up for (plot spoiler heh).
Right analog click melee. Hold button for combos (that too Bungie)
Y and Black cycle through weapons. Yeah, I know... wtf? indeed.
The gameplay and story both rock, the ragdoll physics engine makes for some hilarious kills, especially via explosives/melee attacks. That said the enemy AI is pretty eh, but that's made up for by the fact that they usually attack in swarms.
The story is the usual sci-fi stuff. Government alien experiments, secret third party with their own agendas. Something goes wrong and the shit hits the fan! You and your team are sent into Area 51 to rescue a haz-mat team who went missing. While at first it's a seemingly normal recovery mission fan meets shit and people start going crazy/mutating killing each other etc. But I won't spoil the plot.
There are also little things that are pretty cool. Your character is voiced by David Duchovny (Mulder woot!), so you definitely get that whole X-Files vibe throughout the game. Also there are some newspapers scattered around in the chaos with headlines like. "President Re-elected" and "Red Sox Win The Curse is Over!" etc very cool.
You'll definitely have fun with it. But as I said, it severly lacks in the replay department. Unless you're like me and just like to shoot the hell outa things!
If you're looking for a nice FPS till something like F.E.A.R. comes out this is pretty good, only problem I have is the AI but other then some stupid AI moments it's pretty good.