a friend had one as recently as 3 years ago--basically you buy a number of miles at about 15-20 cents/mile (as I recall). You used them on flights at will--she would use them on otherwise expensive flights, but could and did almost always do better with advanced purchase (given appropriate notice)

Moral is don't over-buy the tickets--look hard at your travel patterns before committing to a lot of miles.

It also included free Prem Ex status (as I recall)

jl