I suppose now we're up to #635, and it seems fairly stable, I could remove the 'Beta' from the name?
No point rushing these things.
One thing I could easily add (because the past puzzles are stored in the database) would be a little drop-down selector, so that you could revisit earlier puzzles, as well as tackling the (default) puzzle of the day.
I've done no further development on the 'enhanced' version with the themes, pencil marks, etcetera. My excuse is that the simplicity is part of the appeal - there are lots of fancy Sudoku apps out there already, if you want all the bells and whistles.