I'm playing as intended, as well, but that doesn't mean I wouldn't enjoy going back and playing a second time with this. I'd probably enjoy it a lot.
The darkness makes the game for me. Granted my system cannot handle the game, so I play in 640x480 with everything turned off and it looks like Doom 2.
It's been memorable, though. At first I walked off a ledge and fell to my death because I couldn't see the lack of a floor. Now, this was in the first 2 minutes of the game, near the loading dock. I kicked the brightness up after that, but only so I can occasionally see the floor.
I've learned to love the shadows. Camping, which I always hated, becomes thrilling in DM. And single player, damn. I love hearing zombies and sitting under the only light in a room, knowing they can see me but I can't see them. Slowly a silouette appears and you blast it to hell. Or running down a hall, turning, taking a few steps and suddenly seeing two red eyes an inch from your face, it's so dark you ran right into it.
The game is ridiculously easy in the lowest two settings, though. Shotguns send you back 20 feet, imps trash you so you can't aim, but you take next to no damage unless you grenade yourself (oops, I didn't realize how far they'd bounce).
All I ever think, though, is wouldn't night vision be standard that far in the future in that dim a base?