Some puzzles not possible to solve, but has good variety of board sizes
I downloaded this app because I am working on a project dealing with the 15-puzzle for a class I am taking. Only half of the possible ways to arrange the numbers 1-15 create puzzles that are possible to solve. While most 15-puzzle apps generate only solvable puzzles, this app also generates puzzles that are mathematically impossible to complete. This happened to be useful to me since the project I am working on deals with which ways create solvable or unsolvable puzzles. However, for those who wish to simply play the game, this could create a frustrating experience.
This being said, the app provides board sizes from 3x3 up to 6x6, which is larger than many other similar apps offer. I consider this to be a good redeeming feature of the app.