I remember this as being part of the short lived (2 seasons) of great or significant plays filmed by famous directors, or, just filmed at all. This was in the series that included "The Iceman Cometh" with Lee Marvin and "The Man In The Glass Booth" with Maximillian Schell. These would come around and you were encouraged to buy a "season pass" to all the filoms in the series. I was in San Diego at the time and the series played there. Sadly, they did not do well and the project was abandoned after the second season. The series was called (I think) the American Film Theater.
I was a young devotee of rock when I saw this and I was fascinated with the movie- a loose framework to perform the songs of Brel (who himself appears and does one of them). I guess it was a kind of filmed cabaret show.
It's great. It predates MTV, of course, and it would make an interesting double feature with "Aria."
So, the praise you may read about it here is justified. Bummer it is not available to rent or own, but I did find the soundtrack and that is something.