The campaign is pretty frustrating, but cheats make the second playthrough a blast.
There's some decent humor in the references to Halo and to some internet memes, but the rest of the humor just looks like it came out of the head of a 13 year old boy. I know that immature humor is half of Duke Nukem, but some of it is just really, really bad. (The wall boobs)
In the end, while the King is back with a few (and only a few) entertaining portions like any fight with the Devastator, there are several portions that just aren't fun. If it weren't Duke, I wouldn't have played it.
I recommend taking advantage of a friend who made the mistake of buying the game over actually buying it yourself. If you want The King for real, play Duke Nukem 3D.
EDIT: After playing through the campaign a second time with infinite ammo and invincibility enabled, as well as insta-gib on half the time, the campaign is actually quite fun. Using the Devastator through every level after you get it the first time in the real game makes the game surprisingly fun.
Without cheats on, I give the game a 4.5/10, but with cheats on, it's ridiculously fun once you know what to do. With cheats on, I give it a 7/10; it's still flawed, but the over-the-top action and lame humor make it entertaining. That still doesn't really make it buy-worthy to me.