The movie was pieced together from 100 hours of footage shot at rehearsals for the star's ill-fated comeback shows.
Fan Darryn Wade, in Los Angeles, said: "Judging by that film, it would have been the best show of all time."
But Kasian Daley, 17, at the London premiere, said: "It's an easy way for them to make money off his name."
And Mustapha Duggash, 18, from Nigeria, also at the London screening, said: "I believe it's a nice tribute but the main motivation is the money."
Jackson's brothers attended the screening in Los Angeles and simultaneous premieres were held in 18 other cities. At that premiere, fan Ed Rahmen said the movie proved the concerts at London's O2 Arena would have been "the best thing he had ever done".
"It was like a film on stage. It was better than any of his early stuff. It was Jackson brought to this time and age."
Neda Allin added: "You can see he was a bit ill, or he didn't look right, but the movie was brilliant."
David Montalvo, who saw the film in New York, said: "I loved seeing him in action again. It's like you were able to see Michael again for the last time, so it was a good chance to say goodbye to him."
Marilyn Morrison, also in New York, said the footage was "excellent".