I went into this movie with some trepidation because I don't have a lot of confidence in film adaptations of classical material. In this case, I'm glad I watched it.
There was some added material - Sir Gawain And The Green Knight, in its original form, is pretty short, so I wasn't surprised. The new stuff was consistent with the source, and mostly served to set up the mythical/spiritual realm that Gawain travels to in his quest. It added to the story - as opposed to distracting from it.
The plot is a meditation on honor and virtue, which is increasingly rare. I found it to be thought-provoking, though I think some familiarity with the original contributed a lot to my enjoyment.
The senery, costuming and dialogue were, if not perfectly accurate to the time, accurate enough to be acceptable. I'd like to see what could be done along these lines with other medieval material. Song Of Roland, perhaps.