I think this is a really good film for Shelley to have done because it shows that she is a much better actor than she got to show in "Cheers". Pretending to be a different kind of person than you are in real life is good acting, and convincingly playing several different kinds of people in one film is great acting. It is true that the medical condition isn't examined as a medical condition but in my opinion that is not the point if making this film - it was just to give the viewer a feeling of what having multiple personalities does to the subject's family and friends, and in my case it succeeded in that endeavour.
When I saw this film I had never seen an episode of "Cheers", and had never heard of Shelley Long, so all I saw was a great actor who for long periods of a rollercoaster story made me believe that it was all real. I loved the salesperson who could almost literally sell anything to anybody, and I especially loved the plate smashing scene. In my experience only a few other TV films are anywhere near as good as this one, the only one that springs to mind being the Amy Irving version of "Anastasia".