I'm now on the level 'Underground Part 2' which is level 20 of 23 and I've been thoroughly enjoying it.
It's based on the Unreal 2 engine and this means both the graphics and gameplay are dated. It's also fairly linear.
Gameplay modes are archaic, it feels like Half Life 1 sometimes, pivotal/weight/acess problem solving is SO 1999 darlinks.
It's also sexist and there are bare boobies in the game. And there is mucho cussing
So that's what's wrong with the game. What's good is that it is a helluva lotta fun and the one-liners from Duke have literally had me laughing out loud sometimes.
The two hardest levels for me so far have been a boss level 'Queen Bitch' and 'Duke Burger part 2' a kitchen obstacle course where you're shrunk.
To sum up: dated, don't play if you're easily offended but a game that's lots of fun and doesn't take itself too seriously.