I have taken the liberty of joining the Wazu zombie mod server by myself and I must say, it is pretty fun. The general balance seemed well, but I'm unable to see if anything will change when other players become zombies as well. Especially on Warp the mode played pretty well, because you're constantly moving in a circular fashion, making it hard to check behind you when zombies are coming from the front, increasing your chance of getting killed because you're not paying attention for a second. The feeling I get from such modes is just great, and I hope it can be included in the game soon. The only thing that's weird is that the bots pick up weapons, but that's nothing major.
If the issue of bots picking up weapons will be fixed and the bot names will be changed to "Zombie", instead of "Bot[0]", "Bot[1]", etc. (though, not sure if that'll work with other players becoming zombies) I believe this is already a pretty well playable mod. Again, I haven't been able to see how it plays with more players, but for a single player it's nicely balanced and the 1-hit killing surely is not too much of a punishment, it makes the mode a lot more fun and feeds the paranoid side of me really well.