I may have only just started, but i'm having a lot of fun so far. It's basically what you see- you're completing a jigsaw, but to change things up a bit you also occasionally get attacked by zombies at the bottom right, so you have to think about your ammo and barricades etc. So that you don't die. It's fairly challenging, actually.
The puzzles are nice and it doesn't have the problem of everything being a similar colour in the jigsaw, the art is pretty good, and it makes a nice change in the pace of the game when you suddenly get attacked by a horde of zombies.
There's also an option when you start each jigsaw to turn off the zombies, in case you really don't want them.
Also, the jigsaw pieces are shaped differently to the usual looking jigsaw pieces, which is an interesting idea and you have to think differently to how the pieces go together.
It's an interesting idea, and I feel it works well.
There also seem to be enough jigsaw puzzles to keep you busy and justify the price, and the devs are adding more for free.