This FPS is much different than Quake 2, where its engine is modded for this game.
You can still go-out-and-blast on Novice or Easy, but at Normal or higher difficulty, you have to think about stealthy approach, talking around, or get your henchmen and gang on enemies.
AI are decent for both your henchmen and enemies.
They try to follow you through vents and ladders(think of the fact that it's released one year after Half-Life), ask you to stop for healing(they regenerate slowly, somehow).
Though you can't bring them between chapters and you can have only two henchmen at once.
If you don't need those henchmen anymore, you can just shoot them in the head and grab their guns dropped.
During combat, some cowards(NPCs) try to run if you have guns better than theirs(like, you point shotgun at a guy with lead pipe) or they're hurt too much(They usually leave blood trails), and not-so-wimpy NPCs will strafe and crouch frequently, making your aim much harder.
The singleplayer is hard, as there's some parts where you can get stuck(like, Chapter 2 have a bad ending scene), or hard to find things and ways out(easier than Xatrix's previous game, Redneck Rampage though).
Overall I liked its difficulty, and its DEMO(contained an Chapter 1, without the scrapyard map and Flamethrower was hidden in the last map) was the FIRST FPS I HAVE EVER PLAYED.
Note : this game contains LOADS OF F-words. I've seen no games with more F-words than Kingpin.