Finally get to do something I always wanted, which is to play through the original Doom, DoomII, and expansions, on high level, and with a secrets map handy.
So far I have to say that the original Doom actually dates very well, gameplay-wise. There is no up or down look, but you don't need it, and the enemies are incredibly fun, requiring very fast reflexes to fight, especially in the numbers they come at you.
Doom was a lot of fun, Doom II is even better, with a far higher enemy count, tougher monsters, and some very ingenious levels, and lots of them.
It's very tactical, very fast (your run speed is like Usain Bolt), and never feels cheap. Your aim is stuck in horizontal, but you must place each shot accurately, and on a lot of enemies you have to unload on them as they keep coming.
It's hi-def, very fast framerate, just a lot of fun. I am very tempted to say DoomII is the best "shooter" ever made.
Haven't played much into Doom III, I'm waiting, but I did go about ten minutes into it. You can just tell that the story is top notch, lots of attention to detail, lots of atmosphere, and while the graphics aren't Killzone 3 or Gears of War 3, etc, they are more than serviceable, the lighting is fantastic, and the framerate is through the roof.
For $40, highly, highly recommended.