Okay, finally got my discs so here's a short summary.
"The Dark Crystal" Superbit is virtually indistiguishable from the original release in terms of picture and audio. There may be a slightly increased level of detail in the picture, but you have to look really hard for it. And the print used to create the video is exactly the same, so all of the dirt and dust that appears on the original release is also on the Superbit. Considering the Superbit release drops the extras from the original, I can't really recommend anyone get "The Dark Crystal" Superbit. Just stick to the original release.
The decision is less clear with the "Labyrinth" Superbit release. The video picture has modest improvements - it definitely shows some more detail, looks sharper, and has less "shimmer" to it in many scenes. The sound is also better - the original only had a Dolby Pro Logic mix, whereas the SuperBit release comes with two 5.1 mixes (a Dolby Digital and a DTS one). The two new 5.1 tracks are nearly identical to each other (although the DTS one is recorded at a louder volume), but they are both have better clarity and range than the original's Pro Logic soundtrack to my ears. The spatial mix is essentially the same, but there's better separation. I thought the original's soundtrack was a little muddy sounding, so this is a welcome improvement.
Should folks get the new version of "Labyrinth"? If you have a good display and a good home theater sound system and you love the movie, you should consider it. But many folks with just an ordinary TV and speakers probably wouldn't notice the difference in picture or video, and once again it drops the nice making-of documentary and trailers that appears on the original release.
For the record, I have a Sony Wega 27" TV that can take advantage of anamorphic enhancement - it's probably not considered a real "home theater" display by enthusiasts, but it's high quality for a 27" TV, and I could definitely see the difference between the video of the two Labyrinth releases.
I hope to write up a longer review, with screenshots showing the differences in picture, eventually.
Hope this helps folks for now, though!
--Scott McGuire