I've only just watched the first movie from my box set, and I am sorry that I bought the Blu Ray. It is the first in my collection to truly disappoint me. The movie is always grainy (more so than the DVD version that a friend has), and the apparent 'upscaling' has if anything reduced the quality. I considered giving it 2 stars as the first and third films themselves are good, but the second film just brings it down (yes I have seen them, I just haven't watched the blu ray of the second and third films).
If you want these films, buy the DVD. Cheaper, and probably better picture quality.