A decent distraction
Yet not much to come back to. As others have said the pistol is about the only weapon needed to beat the game, and even with minimum upgrades is a far more effective weapon than the more expensive ones. In the end I even felt punished for using anything but the pistol, despite half of my upgrade points not being devoted to it.
The game could of also used more of a money sink. Perhaps trading the upgrade points to upgrade costs, making players choose between upgrading a weapon or buying new ones with what they've earned (it might also help to remove the pistols dominance somewhat). Grenades too would of been a nice thing to see implemented in the game.
Otherwise, it made for a nice 30-minute time waster, I just wish there was a little more to do with the cash earned.