some said it's truly hard, some disagree, try for fun youself, see how long takes you....

A Finnish mathematician on Monday (11/7/06)

http://www.usatoday.com/news/offbeat/2006-11-06-sudoku_x.htm

claimed he had created the world's hardest sudoku puzzle, a brain-teaser which required three months' work and a billion combinations to produce.
"AI Escargot is the most difficult sudoku-puzzle known so far," the puzzle's 37-year-old creator and applied mathematician Arto Inkala told AFP.