I developed a system with dots and sometimes little numbers (I think? I don't even quite remember it now) to track the possibilities that worked really well -- I could do advanced puzzles in pen, without ever guessing and being forced to backtrack.
Sudoku online was broken for me, though, because I couldn't use my method.
Pen/paper is a great interface for Sudoku -- very fast and flexible. Are there actually virtual interfaces that beat it, or is the only advantage portability (I'd have my phone at the doctor's office, but I'd probably forgot to bring a Sudoku book)?
I watched the video for this app -- is there support for any method of tracking number possibilities in a given square, or is the only option "make a reasonable guess and see if your score goes up or down"?
The latter is all I saw, and that's a horrible way to do Sudoku, no matter how pretty the graphics are.
It's interesting as a logic puzzle, not as a guessing game.