I have it and played it before I came to work. It's pretty awesome and there's no junk key maps like usual FPSs, just exactly what you need. That's a first.
On the system in my sig I have to switch it to low quality shadows and turn off AA etc.. completely and then it's playable, otherwise it's lag city but for me that's true with any Source based game. So expect performance to be in line with any other source based game you may have. If you don't have any Source based games then expect this one to be a hog like the rest.
You pretty much need top of the line in everything to run it with playable FPS with everything turned on at max resolution.
As far as gameplay goes it's pretty tuff but you can play it super easy as well to start off. Instead of playing Easy Medium Hard it's a slider. Plus there's a second slider that allows you to dictate to the game when to give you a hand.
Basicly that second slider means how many deaths in an area before it stops spawning various enemies to reduce the overall difficulty.
I set both to half way and it gives me a pretty good challenge and I die lots in tuff spots. You can easily take enemies out but you yourself could also be easily taken out if someone has a shotgun at point blank range or tosses a nade.
I don't know how AI is really cause I find enemies stupid in all games. I mean what moron would run at you even with a shotgun. Just wait and camp. Yet they run. Although I did notice enemies do hide beyind objects for protection especially if a buddy or two gets killed but that doesn't seem to effect any reinforcements that seem to always run towards you. As I said AI in all games is stupid, even the best AI.
So don't expect some sort of miracle in that department. Pretty much the same as F.E.A.R.
The gameplay and story is pretty fun but I only had just under an hour to play so I didn't get very far but what I saw I liked. I'll definately buy the 2nd Episode to see what happens next. It's worth it.