Finished it last night on UV difficulty and after getting thru the first couple of levels, the game finds its groove pretty quickly and doesn't let up til the very end. After you get enough equipment to deal with most of the encounters and after you figure out how to deal with the marauder, I had only one complaint that persisted with me thru the entire game, and that is during a fight getting stuck on something and not being able to move. Running around the battlefield and reassessing your position, dealing with the enemies and their weak points is a great way to not die. Every once in awhile, I'd dash and get stuck on some sort of level geometry, a zombie i didn't see that came around the corner or.....something and get stuck in the ground. At the higher difficulties, not moving for that brief moment might as well mean I lose a 1up because that instantly made everyone in the arena hone and and destroy doom guy. That was the only time I'd ever lose, and since the battle speed is so high, I could never figure out if it was my fault or not.
Another thing I've been thinking about with some of the complaints about having too little ammo or the battles being too hard and what not is what control method people were using to play the game with. I played with both a controller and a keyboard and I cannot fathom playing some of the later levels with a controller. It's just not precise enough to deal with the level of speed you need to pull off weakness shots and what not. Weapon switching, pulling off precise sniping whilst running around the battle field etc. My enjoyment went up considerably after I put the controller down for good and just used a mouse.