I saw the PS/SS first, with a nephew and the kids. Then the kids wanted to see the COS as a birthday celebration for the daughter. They were around kindergarten /first grade age. So I bought them the first two books, and of course they wanted the third book when it came out.
So I bought that, then read it, and it amused me greatly. I then backtracked and read the original and the second. The first two movies were predictable, IMO. After the POA, I could get into it. It seemed more real, more things don't always have a happy ending, it was darker.
Since the second movie, it has been a traditon in our household to go see the current movie. I insist the book be read first, however.
We all know how it's going to end round my house, seeing it onscreen will be make it final. Actually have a meetup to view it with friends, and kids friends, and kid's friends parents, we are such nerds.

I also preferred the books to the movies. POA and OOP were my favorite books.