I have the full version and every achievement on it and I would say to you that it's definitely worth it. It's pretty fun to play, it can get repetitive at times, but it's pretty sweet. There are seven gameplay modes (the standard 55 Days mode, Blackout mode, All Weapons mode, Chainsaw Only mode and the infamous 7 Days of Hell mode and two others that I'm too lazy to look up, a lot of enemy types, about twelve different weapons excluding your basic infinite ammo assault rifle and it has four-player co-op for every mode.
The most time consuming thing you'll do in ZA is 7 Days of Hell. On Day 5, 6 and 7, that single day can take you a couple of hours to complete, and you will fight huge waves of every zombie type in the last day.
In my opinion, ZA is a fun twin stick shooter that should last you a fair while.
The single funniest moment in this game for me is when you finish a day playing as Jimmy 86, occasionally, he might say "Oh. Yeah. I am the man. Who is. Your Daddy. Danger. All zombies. Danger." in a robot voice.