I just finished the game last night on the hardest difficulty. There's plenty of action, loads of humor, and everything works out pretty well. Duke is completely over the top, and as far as the personality goes, the Duke absolutely delivers. The game takes a funny, if not ironic, shot at Halo in one scene. There's even an updated reference to the old "That's one Doomed space marine" joke, which was pretty funny.
The weapons all felt pretty decent. The two-weapon system, surprisingly enough, doesn't seem to be as bad as I thought. Usually there's another gun laying around that's appropriate for the situation, and I have run out of ammo once where I was down to just fists and no pipe bombs, wondering if I was going to get killed. The lack of health kits has been replaced a bit with ammo management in some spots. The weapon limit seemed to steer me away from simply "playing with" the weapons, as I routinely passed over more specialized tools like the Freeze Ray and Shrinking Gun. Trip mines seem to work best for me tossed like a grenade so an enemy can do something stupid or for extra bang in a fight when set off by a pipe bomb. What's great is that anything that can be picked up can be used as a thrown weapon. I've killed aliens with some pretty odd stuff.
Enemies are downright groovy. The troopers and assault captains are visually a lot more developed than in DN3D and the special effects on the captains' teleport and jetpack effects are very well done. Pigs are nastier than ever. They're not as comical as in DN3D. They're vicious, tough, and all-around mean. Yes, the assault commanders still poop rockets at you. If you thought bird poop was dangerous, stay away from those guys.
If I have to peg the game on one thing it's lack of quicksave and the old-style slot-based save system. If they had included that on the PC version it would have been huge. I can deal with the checkpoints but I've never been a fan of that system. Consolitis there, but I don't consider it game-breaking. There are some slower spots in the game play, but hell, wandering around looking for secrets in DN3D could be pretty slow too, and they don't feel like the break anything to me. DNF is a lot more linear in that regard however.
As for the "mature" content, there is a lot of sexual humor present. There's a lot of... what is the word for it... similarities in some things and resemblance to anatomical structures as well for lack of a better description. There is no option to disable nudity as with DN3D either. It is definitely not a kid's game nor does it pretend to be, and there's also severe language. I don't have a problem with this as I expected it.
I would not say there's anything particularly ground-breaking about the game. There's nothing that hasn't been done before as far as basic game play elements. You do get a lot of fun stuff in the world to play with, and you're rewarded for looking for secrets. I don't think anyone should be expecting this to be "the best game ever made". If the question is, as I think it should be, "Is this a good sequel to DN3D?" The answer to that is YES. It has almost everything I'd want to see in a Duke Nukem game, except for quicksave and maybe being able to carry a few extra guns. I'm pretty pleased with what I've played. It's not perfect, but pretty darn fun. I have no regrets about pre-ordering. Vaporware no more, baby!
Edit: I haven't tried the multiplayer yet, but it looks like it should be a blast. Things like the Holoduke and Jetpack will be particularly interesting. You also get to rack of points in order to decorate your Duke Mansion with funny paintings, statues, and even NPC babes.