Hola Jacqui,

both "es buena" and "está bien" can be used to describe a film as "being good". Even though the film is always good, we can still use estar bien. It is more to do with a general use of "estar" with the adverbs bien/mal (not with being a permanent or non permanent thing) I added an extra note to the lesson explaining how, when talking about films, plays, books (entertaining things) we often say "está bien" or "no está bien" to add the nuance of being good in the sense of being "entertaining, amusing, interesting". 

I hope this clarifies it.

Saludos

Inma