Sir Michael Caine lists this, along with The Swarm (1978) and Ashanti (1979), as one of the worst movies he ever made. This is mainly down to the fact that no one, least of all the audience, knew what it was about.
A critical and commercial disaster. Screenwriter John Fowles laid most of the blame at the feet of director Guy Green (despite Fowles having penned the screenplay). Fowles vowed never to write another screenplay after this (nor did he). The only subsequent adaptation of any of his works was Karel Reisz's The French Lieutenant's Woman (1981), which was adapted by Harold Pinter.
According to William Goldman, in his book Adventures in the Screen Trade (p227), when Peter Sellers was once asked whether he would make any changes to his life if he had the opportunity to do it all over again, Sellers replied he'd do "everything exactly the same, with the exception of watching The Magus (1968)".