Well, i've played it for about 4 hours now, and i think it's freaking awesome.
First, you're right, it is dark, but i think that's perfect. Sure there are times i wonder if there's any duct tape around, but then how are you going to deal with changing weapons and still having a flash light? Maybe they could have instituted battery life with flash lights, but i think it's just fine the way it is. There are some fights where you're in the complete dark, and it's useless firing into the dark and hoping through luck you'll hit something. In those time, you run back out into the light and have them follow you out.
This game was all about atmosphere, about things jumping out at you in the dark, and well... if they're going to have that, they need to make it dark! There are some good freaky moments in this game, with the devil or whatever playing 'jokes' on you, babies crying, etc, it just sets the mood. And it's just more fun in the dark imo. When you enter a new room or corridor, you don't just go barging in, you use your light, scan the area, then baby step in.
The story is just great... the way you enter the world just like another marine, then get caught up in the middle of it, and you need to make your way back to others. I have it on Marine difficulty at the moment (medium), and i think the game is a bit too easy at this level. Monsters are quite easy to kill, although at later levels, you do run low on ammo (i've just sent the transmission).
They replaced the color coded door cards with PDAs, which i've heard some people complaining about having to read emails in a game, but i LOVE it. I think it brings an awesome addition to the game to read people's email, find out security codes for lockers and storage, look at video and hear voice recording of the world we're playing (really, you can learn about why the research station is on Mars, how they're transforming the soil to air, water, fuel, etc down at the atomic level... it's just damn neat).
I have the game at 1920x1200 resolution on medium setting, and it looks awesome (3200XP, 1gb ram, radeon 9800Pro 128mb). I get 30fps, and it drops down to 10fps in heavy fights, which gets me hurt more than it should... i tried playing at 1280x1024, but on my widescreen monitor, that just blows. I'm sure when the fps really starts to hit me, i'll just switch over to my 1800FP and play it at that resolution, but in the meantime, i'm just being extra careful and enjoying the game at that resolution. I can see myself coming back to this game when i upgrade my computer, and trying this game at 1920x1200 on high details with difficulty set to HIGH.
So all in all, a damn good game. I don't play FPSes usually, the last game i can remember enjoying is KOTOR, and before that... Duke Nukem... so you can imagine how many years ago that was (i'm more a RTS, RPG, MMORPG, Adventure gaming guy... i just don't have the twitch finger to succeed in FPS, although this game is a bit easy on Marine imo). But i have to say, i find myself noticing that i've clenched my jaws in more than a few fights, and need to consciously relax.