I read and enjoyed all the Agatha Christies when I was a teenager. Despite this I love these adaptions, and I enjoy the new twists we get in the screenplays.
I have to say that Geraldine McEwan for me is the consummate Miss Marple, as envisaged by Agatha Christie, managing to combine the ideas of sweet little old lady, knitting pink woollies for babies, with intelligence, humour and determination. 'Just passing were you, Miss Marple?' 'Oh, Mr Burton!'
Julia McKenzie is not nearly as successful - have you noticed that her garb is far more severe, with jackets, stiff collars and plain hats rather than the cardigans, and flowery hats of McEwen? Even Joan Hickson, in the earlier series filmed in the 80s and early 90s, does not match EcEwan for authenticity.