I have not bought the box set yet, but the couple of reviews that I've read have been raving. The PQ is supposed to be stellar- "Oz has never looked so beautiful on screen" as one reviewer said. As for the AQ, they added a TrueHD surround track that still relies mainly on the fronts (as any classic movie should), but uses the rears for ambiance and some of the music.
Given that this is one of the first movies I remember watching when I was very little, it is going to be very interesting to see how much it has changed in HD picture and sound.