Addictive zombie killing fun
The zen garden is a nice addition, and the achievements will take many more hours of playing to be fulfilled - if one chooses so.
The almanac and the notes the zombies send throughout the game are humorous and well-written (I chuckled when I clicked the "help" vase out of curiosity and read the contents!).
There is also the option of creating your own zombie avatar which then becomes one of the zombies that carry the flags in the various waves.
Overall, it is nice, simple, fun and addictive.