There are plenty of problems, but it isn't an entirely unpleasant action ride as long as you know what to expect.
The hype built up around the game is what has ultimately killed it. For some reason, the gaming community has gotten it into their heads that Duke Nukem is the savior of their chosen past time. In reality, his games never were original or particularly amazing, and for those that don't agree, think back. Do you remember anything about Duke Nukem 3D past the second level aside from a couple notable one-liners and a final boss on a football field? Face it, it was Doom with strippers so all the lonely teenage boys hailed Duke as a god. Now those teenage boys have grown up and would accept nothing less that the greatest game of all time, a title that it never had a chance at being without changing the basics of the series.
The story picks up shortly after the events of Duke 3D and seemingly disregard any other games in the series. Duke is a national hero enjoying life in his penthouse suite complete with bronze statues of himself, massive home entertainment center where he plays the newly released Duke Nukem Forever game, and a few posters for "Hail to the King" the musical starring himself. On his way to appear on a talk show, the aliens return and he is thrust back into battle against his old foes, and old foes they are, as there weren't any new enemy types other than bosses. The only enemies you will find are Pig Cops, Octabrains, variations of the Assault Troopers, and a few bosses. After twelve years, I would have thought they would have come up with a few different enemy types. The plot progresses as expected, which is to say that nothing really happens other than Duke killing lots of aliens. Since there was no need to hire writers, no one could figure out how to end the game. Their decision was to kill Duke, then after the screen goes black, his voice comes on and says, "What kind of sh*t ending was that? I ain't dead and I'm coming back for more!" Really? You didn't know what kind of ending to put in the game, so you made something up that doesn't even really happen? Just because you make fun of your ending doesn't make it alright to not put any effort into it.
The levels vary greatly, if both locations and quality. For every awesome level such as the fast food joint that Duke has to navigate after being shrunk, there is a terrible one like the alien hive that is literally one green hallway after another for about an hour. The length of the game is decent, clocking in right around 10 hours, but a few levels seemed to have been artificially extended. Numerous times you will find yourself shut into a room and must kill an unspecified number of enemy waves before proceeding. Even the expected strip club level is totally out of place as it only appears as a dream sequence after a boss battle.
Difficulty isn't a gradual increase like most games. You will run into several sections that are incredibly difficult, followed by a few hours of easy gameplay. Actually, the game gets easier as you progress due to the way Duke's health is implemented. The Ego bar (basically the same as a health bar) is surprisingly short at the beginning of the game but can be increased in a few ways. Interacting with objects will give you a boost the first time you find them. Some give only a single point, like throwing a paper airplane. Others give bigger bonuses, such as setting the high score on the pinball machine (about the pinball machine, it is poorly designed, as the game tells you that you lost the ball far before it rolls off the end of the flippers). Defeating bosses give you the biggest boosts. In the beginning, you can only take a few hits before needing to hide and regenerate. It isn't hard to have three times your starting amount of Ego by the end, and since you fight the same enemies throughout the game, things simply get easier.
The graphics are the most dated part of the game. Textures are pretty bad, especially when you get up close to anything. Many of the levels are bland and unmemorable. Enemies look pretty intimidating, but simply pop into the air a few inches when they die and turn into rag-dolls. Walking onto their corpses will then cause the to shutter like they are having seizures until you walk away. Another significant problem lies in the loading screens. They take a while at the beginning of a level, but this is to be expected. However, they take just as long when you die, which may happen alot in some of the platforming sections. The platforming in the game would have been much more tolerable of you didn't have a thirty second wait every time you missed a jump.
The game's humor is hit and miss, and unfortunately misses a bit more often. Things got off to a rocky start when the game opens with you using the Devastator for a few minutes and it has 69 shots. Then it is revealed that Duke's penthouse is on the 69th floor. I get it. It's a sex joke, but plastering the number 69 all over isn't funny. Most of the things I did find humorous were references to other games. Early in the game, Duke meets his friend that looks strikingly like Marcus from Gears of War who says to Duke, "I just got back from helping my friend find his wife. Christ, what a f*cking p*ssy." Actually, the parts I got the most laughs from were the game's help text during loading screens. Most of them are helpful, but occasionally something like 'To stay alive longer, avoid enemy bullets' would appear. These blatantly obvious comments when I was expecting something insightful were always good for a chuckle.
Duke Nukem Forever isn't a classic, but it is a fun experience if you can look past the flaws and it is double the length of most shooters out nowadays. Past games in the series had hinted at it, but DNF fully embraces Duke's role a a parody of the FPS genre. There are plenty of problems that should have been ironed out over the last decade, but it isn't an entirely unpleasant action ride as long as you know what to expect.