Fun but easy...
I had a lot of fun playing this and acctually played this game for quite a bit. Only problem is that once you have a good strategy, the game becomes amazingly easy. My personal strategy was lvl up on speed, fire rate, and shell size. I'd then run around in a circle like a mad man!(dodging the occational zombie that would appear from the walls) After rounding up ALL the zombies(I gave up on lvl 51 because I was tired and bored but it was still satisfying to kill 260 zombies at once.) I'd compact them and let my bullets fly. Easy lvl gains, blah blah blah... Fun game but quite easy to get bored of. Smash tv tho... That was a fun game because no matter what you do you would have opponents that would just make you tremble as you come in to the stage. I think also the fact that in smash tv you were dodging bullets made it extremely hard and exciting. Died Hard just gets boring after so many lvls of the exact same thing.
I think that if you were to add something along the lines of booby traps, bosses, some kind of adventure to it by making you choose a certain elevator to go into or something like that would enhance this game to maybe it's full potiencial.