I must echo everyone's comments thus far. Be grateful to be alive! I would def complain, but to the FAA and not to DL. Thank the DL pilot for realizing the pistake before it was too late. To my great chagrin, the company has been booking me on UA a good bit recently, but the silver lining is Ch 9 (for those who are unaware, Ch. 9 on the UA IFE is the ATC channel). I love to listen to it! I am addicted! I flew TUL-DEN-TUL this weekend, and was absolutely shocked at some of the things I heard on that channel. When monitoring Denver Center on our initial decent, I heard the controller clear another aircraft to a particular altitude and speed. As is protocol, the pilot repeated the instructions but stated altitude first then speed. The controller corrected him saying that speed should be stated first and the pilot responded with, "whatever, just shutup!" On my return today, several times the controller asked, "now what was your speed again?" and one time he said, "UA480 decend and maintain FL350" to which our Pilot responded "We are already at FL350 UA480." That's scary when the controller doesn't know your altitude and speed!

On a side note, I head this same controller as a DL pilot "what's you Mach Number DL966?" to which the captain responded "mach 0.875." Silence for about 5 seconds and then intense laughter from the controller and then the DL pilot "Correction: Mach 0.785."