I started as a casual user (The i daily puzzle) but am now entirely hooked on the hard stuff - like crack cocaine but more addictive ...
I've got an excellent app on the iphone/ipad - search for "Sudoku Killer" - it's the one subtitled "puzzles for your iphone".
It's free to try - and only £5 or so for a huge library of puzzles - I'm well over 200 puzzles into both the Hard and Extra Hard categories - and even flirting with the 'Impossibles'.
I fear I may be getting a bit obsessive - apparently I have been neglecting my culinary duties - the phrase "not fish fingers again!" has been heard and a yellow card placed upon my desk ...