I had issues with the demo too...But hey, the full version is fine, you should grab it, the site actually has pretty affordable packages. (
www.gopostal.com ) have a look.
I just played thru a stage where you had to fend off a zombie attack on a house.
I shit you not, I hadda kill 80 zombies. RWS should consider working with a zombie title, I'm sure they could do it well.
Here's some more zombies.