Very addictive!
I had a lot of fun playing this. The whole linearity of it (and I don't mean that it's a side scroller, i mean that you don't choose your own weapons) was rather discouraging, but I got by it. It was just very fun, mindless bloodshed. I did, however, dislike the alien stuff... that seemed a little lame to me. I know you need a way to explain the zombies, but come on. I just hate it when a good game is marred by a bad story. No story is better than a bad story.
Still, I played through it, and had fun doing it. So, good job!