I had a quick chat with a Lufthansa 747-8I pilot recently on my way home (I'm still working on a writeup of the flight). Landing in LAX, I asked to go see the cockpit (which I always feel a little stupid doing as a grown man, but screw it), and the crew gratiously invited me upstairs. In the cabin were the captain and the much younger first officer, still shutting the plane down.
The first thing I noticed is the cockpit looked rather old. I was expecting a much more drastic update from the 744, but I couldn't really tell the difference.
The captain was friendly but seemed disengaged from his job (as opposed to the KLM pilot I talked to last year who loved talking about the MD-11). He didn't know anything about a top off order, and he didn't even know how many 748s were in the fleet. I pretended like I didn't either. I asked him about efficiency, and he said the fuel burn was about the same as the 744, and the efficiency was due to the airframe stretch and the additional passengers.
I've heard two stories on this subject. One Lufthansa 748 pilot on Airliners.net has said that the 748 uses noticebly less fuel on a stage than the 744, but this pilot said it's about the same. If it is the latter, it's disappointing. I would expect the 748, with it's four state of the art GEnx-2B engines, to burn less fuel than the old 744 engines. Otherwise, what was the point of the new engines? If they have to work harder to carry the stretch, that seems like a bad return on the more efficient engines.