Originally Posted by
ONTRandy
UA, AA, and US SWUs are good on most, if not all, fares. I suspect that CO will soon offer similar. DL PMUs are good only on Y, B, or M international fares. Price out the difference and you will understand.
Plus DL seems to have raised the prices for "M" fares to Europe substantially. Another big documented issue is "Z" class which is needed for the upgrade. Hence you pay way more up front and in many cases hope DL opens up inventory.
Many here have used the letters PMU to mean Pretty Much Useless. With AA, which I'm familiar, you can gift them to someone else and you do not see many people talking about them going to waste. AA also allows them on all fare classed. Read how many people talk about throwing out PMU's from DL.