I'm not sure Sudoku is really ever fun to play. It's more of a litmus test to see how well you've developed your logic. And it can be satisfying when you get a good answer.
I think Sudoku can be very fun to play with new rules, when you begin thinking in higher abstractions. I'm a big fan of anti-knight rules.
I look forward everyday to see what new puzzles are out on the Cracking-the-cryptic channel.
IMO, your use of the word 'satisfying' is to me the definition of fun!
Negative constraints like anti-knight significantly improve my enjoyment of Sudoku. They make it more likely that puzzles don't devolve to straight Sudoku near the end.
I think regular sudoku is fun.
On my phone, I've got 566 hours logged in 7700 games. I think it's fun :)
Isn't it a bit presumptuous to assume that it is “never” fun to play just because you don't find it fun?