My experience -- though from a much smaller number of airports -- is similar to that of fimo. I can recall being asked to show my carryon to the checkin agent only once, back in my pre-FF days, but I think that was only to check whether I could lift it and show it to the agent without much effort. And I suspect (judging from his scowl) the agent was having a grumpy day.

My carryon is usually around 10-12 kg.