I've done really hard ones where you are left with boxes that you
have to guess. Pretty much everything is left as it can be this or this. I go with the rule, nothing is every easy.
Then you get to the really hard ones that are like 5 sudokus in 1
or weird shapes that split each box into triangles or a hexagonal sudoku xD
and also sudoku which uses the alphabet
there are also those 3D cube sudokus too

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I was bored at work...
here's my solution to the Sudoku
http://img50.imageshack.us/my.php?image=sudokuwr6.jpg