Whoa.. be careful with the answers.

There are at least three different ways to upgrade someone else and the answers are slightly different for each.

Segment Upgrades - These are for you and you only. Actually, most users will tell you that some are for you, and most of them get thrown away.

Companion Upgrades - these are limited in availability, are paper certificates and fairly easy to use. But they are for a COMPANION ON YOUR FLIGHT.

Miles Upgrades - These actually use Skymiles. Someone needs to clarify exactly when and how you can use these. I think I know, but not positive. I do know you can give regular tickets to family, friends or even strangers, as long as you do not sell them.