I've done this lots of times. What a treat for everyone! Why anyone would be pissed off is a mystery to me. Still, there is that issue of extra fuel burn and straying from the anticipated flight level. What were those pilots thinking?!

The best airborne Denali tour I ever had was on a United stretch 8 between FAI and ANC on a moonlit night in February. The DC-8's huge windows allowed us a superb view of the mountain under a full moon. Magical.