Grapes,
I've heard figures of 10% being quoted for well designed winglets - whether they actually work is another thing! During my degree, however, the principle has been explained to me and it seems fesable. Only of real benefit to an a/c in the cruise, and thus, better fitted to aircraft with long ranges.
As for the 777, I don't really know why winglets are not used - this is something I have thought about myself. However, it must be appreciated that the 747 wing design is 1960's technology, whilst te 777 is very much later - prehaps thats the key?? Anyone know?
Cheers,
Cuban_8