Alright, after about 3 months of playing this game, I can give an honest review: 4/5. It has an awesome developer and an awesome idea in theory, but when played out it has its issues. It's fun when you get a good community, but all the douches that just camp on GaT make the game less fun; the crossbow is possibly an even worse long range weapon for thieves. All in all, this is a fun game; it simply gets boring once you realize that the amount of fun you can have is related to whether or not the people you're playing with are interested in playing fair. I will update my review when the developers update again, however.
A lot of times when I'm playing Zombie Rush the Guards just camp all the time, and it's really hard for the Zombies to even get close to the brain. Maybe the brain could randomly switch location every 30 seconds or so, so the Zombies aren't overwhelmed?
Awesome game, I had lots of fun with it. Like heycoolguy suggested, a hardmode would be nice to increase replay value. And maybe a survival mode, sort of like an 'all you can kill' mode where the enemies keep coming.
It's always nice to see a game where it's obvious that the developers are interested in constantly improving it with awesome updates, and actually bother to respond to comments. Five stars and favorited.