That's a lot of fun for very few bucks!
If you're into shooters then I think it's worth a recommendation. It's totally straight forward, coop survival pitting six guys with seven classes to choose from, against waves of zombies that increase in difficulty, culminating with a boss fight on the last wave.
It's easy to get sucked into playing the various classes and mastering the different guns, but it's definitely only for people who know they love shooters, and don't expect a very deep or clever experience.
It's cooperative zombie butchery, by way of so many guns, and maybe a flamethrower or katana thrown in for good measure. Nothing more, nothing less.