We are comtemplating our next one, have driven the GL in the 350,450 and 550 versions. I think we can get the 350 for low 70's without too much problem, and the Yukon XL denali we could not get for 70K, we offered that exact figure a week ago and they basicaly told us no way. List was 77 with the one on the floor.
Our 2001 yukon has cost 2-3 times more to keep running and my wife's 2002 ML500. Her ML has 198000 miles and drives like the day we bought it. I have erplaced the radio about 3 years ago and that is the only repair in the last 88K miles. At about 110K or so we did replace some motor mounts and a belt but that I think are all wear items. I cannot say they cost more to maintain over the long haul.
We just dont think the GL will handle all the needs we have for hauling so planing on some combination of a Yukon XL and a smaller X5 or ML.
GL outsold Escalade and Navigator combined last year so it is a viable option. I personally did not like the lack of steering feel and road feel, but the yukon doesnt have much more. I liked the 550 better but it was in the 90's and the mileage was 12-13. Not going there. The 450 was just boring to me. One of the car magazine has a great article this month on the GL. In their short test period they logged 40K miles and everyone wanted to road trip it, but they were not enamored with the driving experience. They summed it up well though by saying you cant teach a buffalo to dance (their test model was brown)