All the Sonys I have seen (325, 525, 725, 735) exhibit layer change - the one on the double sided NON special edition Armageddon is particularly bad on my old 525 - although my soon-to-be-replaced Samsung doesn't seem so bad at this - everything else about the Samsung is sluggish instead
I believe that Superbit DVDs still have a layer change depending on how long the film is - I'd expect my Gladiator Superbit to exhibit a change somewhere - although I haven't got round to playing it yet.
My DVD drive on my PC (3 years old) exhibits no layer change slowdown either - I find it difficult to believe that companies can be so penny pinching as to not put a meg of RAM in there so they can buffer the data during a layer change - I believe a Hitachi model from a couple of years ago did this...
Certainly, currently your average DVD disc exhibits the same "feature" that killled off 8track cartridges - that damn split in the middle