Very good application. I second the suggestion by Garfield Arbuckle about the missing-filler is something I'd like to see also.
The only real problem I've encountered is that very easy puzzles are often much harder than easy puzzles, even statistics wise. Had several with 2-5 guess-squares according to stats.

Very nice, but it would be even better if it supported different layouts, like the portrait mode in Thumb Sudoku.