Has anybody ever used any of these? What for? Was it worth it?

Yeah, sometimes they have rebuy GC tourneys to a bigger tourney, such as the 100+9 75k gtd. They are really soft and there is a considerable overlay usually (like 10 seats, 140 runners for the 4d rebuy a couple weeks ago). Plus, you can buy in up to an hour late, so you can wait to see exactly what kind of overlay you will get. When I bought in to that one, there were only 74 left, a number of people still short, and its a satellite so you don't have to win - just place top 10 or whatever.

The same thing happens with gold chip tourneys.

Sometimes the higher value cards are redeemable for straight cash, or for direct entry into a freeroll. You just have to remember to check every day, and be available to play them at a pretty inconvenient time for a working stiff.