I am PM by way of an affiliate (I am Aeromexico Platinum), and my Delta experience has been pleasantly surprising. I did two trips from NY on Delta. One to Charlotte, upgraded (with wife) to business both ways. Flying to ORD got offered earlier flight, which was great as my original itinerary got delayed shortly after. And the few times I have interacted with agents at the counter (they provide supplemental check-in assistance to AeroMexico when things go wrong) they were fantastic, and thanked me for being platinum - something that Aeromexico never does).
That being said, my only odd experience was being
#6 on upgrade list for ORD-LGA a day before the flight, then finding myself bumped down to
#15 by the time the flight was boarding. Are there really that many Diamond levels on that route? Or was I being bumped in favor of "true" Delta PM passengers? Either way, I am generally pleased by the way they treat their "step-children"