Yeah, by the 5th movie they just assumed that people who came to see it had a little background going in. But I don't think that's a cardinal sin -- the way they slashed enormous bits out of the books for several of the chapters was probably the biggest sin. But then every book could have ended up being two movies each otherwise...
Of the 8 movies, the ones I end up re-watching more often are 1, 4, 5, and 8. I didn't like 2 at all; 3 (Cuaron) was great but it ended up being such an island stylistically; 6 and 7 were good but they were downers as well.
7 and 8 still get played all the time on cable so they're just there.