alt Hacker NewsXanadu was a terrible movie, but the soundtrack was a critical and commercial success -- it went to number one in 11 countries, was certified double platinum in the US and had six charting singles, some of which still get radio airplay over four decades later. (And all the songs on the soundtrack were written for the movie; it wasn't a collection of already-existing pop songs.)
Xanadu is not even really a terrible movie. Terrible movies are worth seeing, at least once, because of how awful they are. I've seen at least three disco movies on roller skates, (roller skates) released at that time. The Apple is certainly terrible; I think it was made to cash in on West Germany film subsidies (the same subsidies that paid for The Neverending Story), even the songs are charmingly terrible. Xanadu has a great soundtrack, but the movie is just boring. Skatetown, U.S.A. is probably terrible, but I'd need to watch it again and I don't want to. :P I haven't seen Roller Boogie.
That said, I would absolutely buy (and wear) a Xanadu jacket if I saw one for a reasonable price.