I've used about 1/2 of the miles I've accumulated over my liftime--which is a pittance compared to your average FTer (I just started flying a lot, and worrying about ff miles a couple of years ago.)

I've used them for 3 things:

-quick last minute trips to see family

-int'l J class tickets on which I wouldn't shell out the $$$$$$

-tickets/upgrades for family and friends--upgrades/tickets make great gifts as they make people really happy and cost very little out of pocket.