Never seen a single one of these. Are they worth it?
The first two I liked but didn't love. They captured the world perfectly, but the acting was a little overbearing in parts. Then the third one hit and it was wonderous. The acting was leaps and bounds better, the effects didn't stand out as much (i.e. they blended in better with the background thus making the concept of a magical world way more believable) and now the fourth one which was also fantastic.
As someone who has only read the first book, I'm approaching each film as a film, not as a book turned into a movie. When I see them, I get the whole story and appreciate how they condense a lot of backstory into either a single line or a visual. I'd say pick them up because they are overall terrific fun. The casts are brilliant.
All hail Alan Rickman and now Brenden Gleeson as Snape and now Mad-Eyed Moody are my two favorite characters in the series.
Also, I loved how they did Voldemort in the fourth one. Ralph Fiennes adds such theatrical menace to the character that his imperious nature is awesome to behold. Didn't like his facial make-up a whole lot though.