I got my hands on F.E.A.R. yesterday, and had beaten it by midnight. Overall, I'd say the game has about ten hours of gameplay, which gets pretty action-packed after about forty-five minutes and doesn't let-up until you finish the ending sequence.
I will say that if you're any "good" at FPSs at all, then you should probably play through the game on Hard or, maybe, even Extreme difficulty. The slow-mo in the game makes the enemies pretty damn easy, and after playing through it on hard, I really wish I had up'd the difficulty level to make the single-player portion a bit more intense. Even at hard difficulty (there are: easy, normal, hard, and extreme available from the get-go), the game was just way too easy. Over the course of ten hours, I'd say I only died about four times. Also, once you beat the game once, it'll take a good bit of will to play through it again, it's just not the kind of game that really lends itself to multiple run-throughs; especially with the lengthy "introduction."
As for the story, it's nothing spectacular, but what is there is presented in an incredible way. I had a good deal of fun making my way through the game and picking up a number of vary DOOM 3-esque audio clips that helped to set the mood of the game overall. Towards the end of the game, though, you really have to pay attention. A lot happens in a fairly short time, and some thing in my run through the end left me feeling a bit... Confused. Might've just been the fact that I had people watching the ending sequence while I played it, thus maybe forcing me to miss a key bit of information or two.
Also, disappointingly for me, I just didn't find the game that scary. I only jumped once or twice (compared to DOOM 3 and The Suffering, which made me damn near wet my pants a fair number of times), and the best scares in the game were ruined for me from the single-player demo. Also, if you played the single-player demo, the level you played is split up throughout the course of the third or fourth level, so don't go in expecting to be greatly familiar with a large stretch of gameplay. This is a surprise I was happy to see.
And, Monolith, for the love of God. CALM THE GRATE LOVE. So. Many. Grates. So. Many. Ducts. It hurts.
Overall: 90%+ rating. Despite any qualms I may have about the game, the combat and mood is just absolutely incredible. It's a fucking blast to play through.
Pretties: