You would think that if the winglet has some effect on drag, it would be noticeable to the pilot when flying with one removed, ie some evidence of yaw, instability on approach or takeoff etc. But apparently not, and one flight I saw with a missing winglet (London to Singapore) actually burned less than flight-planned fuel. And the MEL allows flight with only one?
Made me wonder if the winglet (at least on the 744) was worth the effort.