I also just picked up D3 this morning, and have plunked a few hours into it already.
I've been pretty impressed from the get-go; the atmosphere is great, the graphics are awesome and the gameplay is solid - very reminiscent of the original Doom, except with the brightness turned down (a bit too) low.
Right now, it seems like a solid 91 or 92/100 or so on my scale; an awesome game, but not a "best of all time" caliber game, either. Halo, for it's day, was better by a good margin.
Some of my impressions, thus far:
-Plot is adequate. They spent enough time in the beginning to give it a bit of story, but (fortunately) didn't drag it out into an hour-long affair. Right when I picked up the pistol and started shooting was about the time when I started thinking "alright, let's start killing stuff already."
-The machine gun sucks. I've come to realize, that at the point where I am in the game (the 1-3 hour mark), whenever I get jumped by a group of guys and have the machine gun on, they get a few good hits in before I can drop 'em. Not good. I've found that keeping the shotgun out most of the time works best - if any slow or dumb monsters comes your way, switch to the pistol or machine gun to conserve shotgun ammo.
-The game is a bit too dark. It was great for the first hour, but now it's getting to the point where some sections have total blackness, and it's annoying having to switch from the flashlight to a gun to fire into the darkness, then back to the flashlight again. At the 2 hour mark or so, the thought of finding some damn duct tape became quite overwhelming

. Also, for a marine-issue flashlight, it stinks something awful. You'd think that as marines, they'd be prepared and realize that for some reason this facility has lighting issues, and give you a kick-@ss flashlight. Instead, you get a mediocre flashlight. Maybe that's how it lasts forever...
Also, I think the darkness detracts from the graphics a bit - if it was a bit brighter, we'd see just how good some of these monsters look!
-The graphics themselves are amazing. Monsters look great, the environment looks great. I like the cool "Blade-esque" way the monsters disappear when they die too.
-The level design is adequate. I don't get complaints about it being too claustrophobic, etc. It's very similar to Quake/ Quake2, and I personally like the way it plays. This isn't Half Life, and it certainly isn't Far Cry - it's Doom!!
Basically, this game has met my expectations - the graphics engine is revolutionary, the gamplay is evolutionary.