Not bad. Two problems.
The first is pretty obvious - the graphics. I see no reason to go into it, but they could simply be better.
The second, and far worse, is that aiming at guys on the very edge of your screen can be...well...a problem. More often than not I find myself moving the mouse back and forth, hoping I end up shooting the bad guy at some point and not clicking out of the window. Points like that sorta suck all the fun out of the game.
The music was a good choice though.