If one is saving miles for retirement travel then I would say that FF miles are pyramid scheme. I personally try to use miles at the same speed as I earn them. I always try to remember that:

1. I do not earn interest on the miles that are sitting on account
2. The miles are devalued when airlines change their redemption rates yearly

December I booked one BF return LAX-GIG on Continental for 90K miles and then used my UA miles on Varig for three intra Brazil business class tickets 30K/each. Excellent use of miles in my opion.