This app is a lot of fun and Im sure youll get many many hours of chess enjoyment from it.
However, you are playing against a chess engine and not a script, and the chess Engine does not always respond the same way to the same moves, which in some cases makes solving a puzzle impossible. In other cases, the engine responds randomly but its still possible to find the solution.
Im up to mate in six and Ive seen this behavior several times, and it can be very frustrating if you spent a long time on a particular puzzle.
Like I said, though, this app is a lot of fun and if you like chess puzzles its well worth your time and money.
doomsdayZen about Chess Puzzles: World Champions