I played a decent amount of Duke 3D back in the day, so I do know where it's coming from. I also played the demo of DNF when it first went up, mostly out of morbid curiosity and because I happened to have an access code from Borderlands. It was the least fun I've had playing a game in quite some time. There were a handful of mildly amusing moments, but most of the humor wasn't actually funny, and I generally like dick jokes. It's possible they saved the good stuff for the full game, I guess.
It seems very unlikely that the gameplay is any different in the full game, though, considering that everything I've heard about it sounds exactly like what I saw in the demo: awful. The game is ugly, the levels are linear, Duke is sluggish, he can only carry two weapons with limited ammo, he dies nearly instantly if he doesn't cower behind cover (how manly is that?), he has regenerating health, load times are painful... Should I go on? It plays like all the worst parts of old FPSes combined with all the worst parts of new ones.
I was hoping for a retro FPS experience, charging in at 40 mph on foot soaking up bullets and mowing down hordes of enemies with creative weapons, along with some over-the-top manliness and boobies, kind of like Duke 3D. Instead it felt more like a half-assed Duke Nukem mod for Call of Halo or something.