I had around 500k unused miles in my account because the last thing I wanted on vacation after flying back and forth each week to different places was to go fly somewhere.
My vacations were usually cross country road trips. My wife and I would rent a nice car, pile the trunk with luggage and just hit the road stopping whenever we wanted. It was the perfect vacation from an overly structured life.
Within a few months of me no longer consulting on the road like that, I was alerted that United was buying Continental, and I'm sure in that press release I didn't read, it probably said something about all my miles, but I didn't pay attention to it until about a year later when United told me my miles were expiring.
So I logged to trade those miles in for a couple tickets to somewhere far far away, and saw my 500k miles from Continental had be devalued to 50k miles at United. Worth at the time 2 domestic round trips if I paid $150 per ticket to redeem them before they expired.
So my miles that used to get me into lounges, and first class upgrades, and basically free flights to almost anywhere in the world got me about a 20% discount on 2 round trip tickets to Denver...
That was the last time I flew United unless it was the cheapest "real" option (I refuse Spirit and Allegiant).