Positioning back from long haul with BA about four years ago, those nice people refused my flight bag as carry on because it didn't fit their box. Being reluctant to entrust my licence, company laptop etc to the hold, I ended up carrying them onboard in a duty free carrier bag. Ever since, I have used a rucksack. It squashes into any size,shape the jobsworths provide.
Also useful when the sun is shining and I go to work on the Triumph
