The repeatability is a let down, of course. In cases of more than one solution, the s/w decides to recognize as correct only the one that conforms to the repeating pattern.

In addition to the already mentioned repeating pattern, there is also a very easy brute force way to solve any of these. Since the number gets locked once it's correct, one can simply enter 1-9 in each square and the correct result will be the one left. This is much less troublesome than the repeating pattern, though, as it is relatively easy to ignore.

One other thing I would find helpful, if the author is considering future versions, is the ability to erase or remove a previously entered digit.

Have checked with the Sudoku helper widget (which is excellent by the way) and once the starting numbers are entered, even on the hardest setting I could do it in about 30secs just by looking at the repeating pattern.

Unfortunately Andrew is right, you only need to look at the above screenshot to see for example the 9154 pattern repeating.
It's a shame. Can we have an update please George?

I am noticing a HUGE flaw. Every time a new board is drawn, the same 3 digit solution/pattern repeats on the board for each column of 3 digits (column 1-3, column 4-6, column 7-9). I mean, if row 1 column 1 starts with "123" then the same "123" is repeated in row 2 or 3 just in either column 4-6 and/or 7-9. This also happens for rows 4-6 and 7-9 (just different set of 3 numbers). This makes it possible for me to solve the board in 2 minutes. It is not totally random. You can …verify this by solving a board and looking for the patterns to repeat.

If someone else verifies this, please post a comment to confirm. More