I use miles exclusively for C/F tickets that I will not pay for out of pocket and 90% of the time these are gifts for family, friends, etc. For example, I recently cashed in 240k miles for C tix on CO EWR<-->BOM (via NWA.com after a DL->NW mileage xfer) for my friend and his wife for their wedding in BOM . No, I would not have purchased these for my friend cash (he's not THAT good of a friend). At a list price on CO.com of $22,500 total, that's about $0.09/mi.

I would say that about 80-90% of my 5M+ miles that I have redeemed in my life fall in this category.

For domestic or intl Y tix, I usually just pay cash as I see these as a total waste of miles.

TFD