Being a sudoku lover I installed this app when I got my first iPhone over two years ago. Ive installed 100s of games since, but this is the only one I still play on a daily basis; and I typically play it multiple times a day.
The ui isnt perfect, and it can be hard tapping the right squares, but given that sudoku requires a 9x9 grid, and the screen is only so big, its hard to imagine how it could be done better.
The only suggestion I have is to support landscape mode. Display the board on the right and the number pad in the left -- my right thumb can select the square while my left thumb taps the digit.
There have been many times when I was sure I had a solution that didnt validate, or a puzzle which appeared to have multiple solutions. But after checking carefully I always found I was wrong. In the probably 1000+ games Ive played, I havent found a bug yet.
Well worth the 99 cents.