Currently Online 
I would say it's worth the purchase. The positives are that the game has great atmosphere through the use of light, shadow, and mood music, the challenge can be a lot of fun playing coop and when you start trying to shoot for the high score the game becomes even more tense since every hit you take will decrease your multiplier.
My main issues are that while playing coop you have to be on the same screen which can make it rough if you don't coordinate extremely well (very hard to do online atm without voice support), you run out of new weapons to buy too quickly for my taste, and the graphics tend to stutter in the more insane situations (mowing down 100 zombies in a single shot).
I really like the meta game of watching all the contries compete to see who's the best zombie killer, I was surprised to see Finland doing so well (3rd behind the US and Canada who I think had a head start based on when PSN releases occur by region).
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