I flew Thomas Cook a few weeks ago and was a bit worried about the 5kg allowance - then more so when the check in girl was weighing everyone's hand luggage in the queue ahead and making people repack stuff in their case. Fortunately she didn't weigh mine, perhaps because it was just a smallish rucksack (not a camera one).
Only problem I've noticed with Easyjet was a friend whose bag did fit the size measurements but sideways - ie the handle was not at the top when placed in the rack. So they tried to insist she checked it in (there had been no problem on the outward flight). This was at the boarding gate, on a day when the winds were 70mph, lots of flights were being cancelled and there was a plane full of rather nervous passengers all being held up by it - the member of staff eventually let her on with the bag.