Out of the 25 trips that I've taken to Europe (counting my mission, twice for work and once with my band for USO), I have paid for nearly half of the trips with miles.

I also had a United Visa and used miles for a trip to Australia.  I switched to the CSR card and haven't cashed in any miles yet.  Put my mom's funeral on the card. (Somebody had to float the money to the family until the insurance came in)

I've had miles cards so long now that I'm pretty sure I don't often buy stuff that I wouldn't without the card.