Depends for each program.

I consider these all good use of miles (awards I have been able to get):

40K U.S.-Europe American Airlines Saver Y Award.
60K U.S. - Argentina American Airlines Saver Y Award.
90 U.S. - Delta Airlines Business Award.

I would say the Delta one is the most valuable not because of the miles/$$ value but just for the fact that I was able to REDEEM SkyMiles!