Superb.
Wicked stuff. A very good play for when your bored. It's not obscenely long, which a lot of defense games end up being, and its not too difficult even in the later stages. By the time you get the Barrett, your pretty much home free, you can nail 4-5 zombies at once if you angle in right. Turn down the volume though, that gun is LOUD. Earlier on, the second gun you get, the Revolver, is pretty good, even mid game. When the SWAT and the Soldier come around, shoot their legs with a strong gun and they'll go down.
And to the maker of the game, just wow. If this is your first then I can't wait for more from you. Well made, well thought out, wonderful graphics, and a great mood to the game in general. Keep up the great work.