Pretty good
The game was pretty fun, but then as soon as you got certain weapons like the Pancor Jackhammer or that on 9000 exp mini gun, the whole game became a synch. Eventually, I got all of the weapons and powerups and spent the rest of the exp I earned just racking up my base HP and guy HP to ridiculous levels so there was basically zero chance for me to lose. Still, it's enjoyable to just run around blowing everything to bits, and I also sort of amused myself by trying to cover every inch of the ground in zombie blood.
Another minor point I want to say is that I think the designs of some of the later enemies (namely the weird mutated crab-looking zombies and those crawling amoeba things) seemed to make to game look too much like some sort of alien invasion, and it mildly detract from the whole zombie-outbreak atmosphere for me.
And does this game ever end or is it literally an "Endless" zombie rampage? Did you actually program a point for the game to end or do the hours and days just keep going up forever? I've been playing for a while now just to see if there's any sort of end, and obviously, there's been nothing so far.