Played about six hours at time of writing review. Steam time shows 1.1 hours, but that's because I have played more in off-line mode. After about four hours play I bought NZA2.
Good game. I liked V2 and NZA is based squarely on that, meaning it's the same engine and some same sets. It looks different even when the sets are the same.
A hundred rounds plus ammo stocks and loads of creepy enemies - nice! They have given the zombies a great sinister and evil characteristic.
There is little to learn. You just jump in without any need to look at a manual, and it's only a little slow at the very start. You get better as you play, and it gets better as it goes along.
Having a blast now. You soon get back into the swing of judging distance and bullet drop so headshots start to flow. Slow-mo take-downs for vital hits. Never get bored of that.
(Paid £2.50 and put down about 2150 zombies so far.)
EDIT: It's about eight hours play time. Very difficult to estimate though since I played offline and can't be sure.