Personally I can't stand cycling with a rucksack, but do use two Ortlieb panniers that are very good (easy to put on/take off, and excellent in wet weather). Unfortunately they've lost a few screws in places, but their customer service has been very good and they sent me replacements quickly and didn't charge.
Of course you have to fir a rack to carry them on which can be a bit of a pain/look sad, but it helps deter thieves from considering your bike as a target.
