I'm nitpicking a little, but preventing wingtip vortices from making it all the way around the tips of the wings isn't the only thing that winglets do. Rather than explaining it yet again
, check out my reply (# 6) to this other thread here
. I know, I always refer back to that post, but I can't be bothered to type it all again.
You're right that tradeoffs must be made when considering incorporating winglets into an aircraft design. The weight is the main negative, and so various things must be factored in to see if it's really worth adding them. Raked wingtips and increased AR
(aspect ratio) also help reduce induced drag, and in some cases those methods are better to use, in some cases they're not.
EDIT: I cleaned up the formatting...
[Edited 2004-03-25 22:47:41]