It's a great game, but missing a couple of features that would make it a 5 star for me.
For starters I'd like the ability to enter a sudoku puzzle from a magazine or newspaper and solve it on my n900.
Secondly, I tend to fill in every empty cell with all the possible markup numbers as part of my process to solve it (and I'm sure many others do the same thing). This is not difficult, but it is very tedious, and it would be great to be able to hit a button and have the application fill in all the markup numbers for you.
I had this feature on a version of sudoku on my Palm and it really made the game enjoyable as it allowed you to concentrate on solving the puzzle rather than systematically counting numbers to see which are missing in each cell.
Believe it or not, having to work out the markup numbers in each cell actually puts me off playing, probably because I am now used to having it done for me.
If these two features can be added I would definitely give it 5 stars. :-)