A nice clear interface greets the user but this non-German speaker was disappointed when he realised there was no English language option. Persevering with the trial, however, produced no encouraging rseults as I filled in 'clue' squares in the matrix. Whatever I tried, the same message "This Sudoku has no or multiple solutions" appeared when the Solve button is pressed. Even when throwing it a set of clues of a known, solvable Sudoku, the same message was displayed.
And this simple solve routine is all the application is advertised as doing...and is all it would do. If only!
As to usability, the method of entering numbers, is difficult. The user has to select a number (1 to 9) and then click the squares on the matrix to enter that number, instead of the easier method used in other applications of this type, where numbers can be typed directly into the matrix.