Great game, but long at the end.
The game was great the first 3 levels, and the 4th with 20 waves was needless to say...increasingly boring and increasingly easy as you acquired all the upgrades... After wave 10 or so, all you really need is a ton of explosive barrels and the lightning gun. The moment a zombie runs near one, shoot out a short burst and it'll pretty much die from the electric current going from the barrel to the zombie. If you're lucky enough, it'll spread out and hit other targets too.
I don't know how many people actually did all 20 waves, but just out of curiosity I decided to go for it. It was long and boring (it felt like it took several hours, I almost fell asleep a few times), but It's got an interesting storyline, but the ending scene just didn't seem like it was worth the effort. If you had 5 waves, increasing in difficulty drastically, then I could see it as more challenging and fun, but 20 waves just ruins it.