Well, it's a game.
Duke Nukem Forever excels at one specific thing: Making the act of alien genocide fun as hell. All of the weapons have a great feel to them, and even the starting pistol packs a punch. Unfortunately, Duke Nukem Forever really wants you to do things other than shoot aliens, and that's it's biggest downfall.
12 years after the events of Duke Nukem 3D, Duke is now a mega-celebrity. Girls want him, guys want to be him. About 5 minutes after pressing start, aliens invade earth, steal our chicks, and Duke has to go kick their ass. You can knock this for being poor storytelling, but it's really all you want or need from Duke Nukem. However, even here the game really wants you to pay attention to things other than the alien ass kicking.
To elaborate, there are long sections of this game where nothing happens. You'll either be jumping from platform to platform, driving a awkward handling vehicle, item fetching, and even just standing around listening to dudes talk. Want to go back to the fun part and kill things? Fuck you, have some boring story.
I wish these issues weren't as severe as other reviews have made them seem, but I can tell you firsthand that at least half of the campaign has nothing to do with the actual act of firing a weapon.
Additionally, everything in Duke Nukem Forever looks shitty. It's somewhere between late PS2 and early 360 in terms of it's graphics, with all the bad bloom and shading that entails. Even the more unique areas like the Duke Burger look and feel boring as a result of this, and that only adds to the general sense of tedium the game carries.
But hey, it can't be all bad. After all, Duke is there to give witty and amusing commentary, right? Well, sort of. Somewhere along the line, the writers forgot what makes Duke the fun and amusing character he is. While some of his lines did elicit a laugh, the more common reaction was a groan. References are poorly made, easy jokes are made even less funny than I thought possible, and...you know, I think the best way to put it would be to just say that if you lose at pinball, Duke says "I've got balls of fail."
It's like they took the Duke we loved and replaced him with a Family Guy loving, meme spouting clone. Only it's worse than that sounds.
On the multiplayer side, Duke Nukem fares slightly better. The sheer focus on combat means things are more enjoyable, but issues with lag, and generally feeling old (not arcadey, not classic, just old) means it's still not worth your time.
Duke Nukem Forever does have some geniunely funny and entertaining parts, and at times the classic Duke Nukem dialogue I loved shines through the meme-riddled bullshit, but overall, this game probably should have stayed dead.