HotD is soooo good. You can buy different weapons and upgrade them. You can also switch between 2 weapons during the game. There are these things you can shoot that slow down time, you can pick up grenades and you can unlock different art or songs. There is a dual pistol and extra mutants option. After you finish the story mode, you can play the director's cut of the game which is longer and has different scenes. You can also move the camera with the bullseye to shoot stuff on the edges of the screen. You can cut this option off if you want to though. Last thing, some parts of the game are just plain funny. I can't mention any of them without spoiling the joke though.
The only bad thing I can think of is that there are not multiple paths to go through like in other HotD games. The language and adult situations do not bother me but I would suggest not playing this game with sensitive people or kids. I had HotD 2 & 3 and Overkill blows it out of the water.