I have yet to ever have to cash in any of my DL miles but in the last month, I had two occasions where I thought I was going to use them for family trips. The first time, the exact flights to Europe had seats available on the dates I wanted. The second time, I was good on the outbound to Seattle, but there were no seats available the day I wanted to return, I would have had to return one day later or earlier (oh the horrors).
I'm sure it is YMMV, but I have had good experiences so far.