I just watched Mysterious Island...and what an incredible upgrade from the inferior DVD version that I own. Great job, Twilight Time!
What struck me the most, having watched my DVD on numerous occasions, was how the framing composition opened up with the proper 1:66 aspect ratio (DVD was 1:85, I believe). There is a sense of vertical space that was not present on the DVD. I didn't expect the AR to have such an impact on the look of the film, but it does.
Also, the 5.1 DTSHD-MA track really sounds great. Not only does it bring out the Herrmann score in ways that I've never heard before, but there is some actual ambience going on in the rear speakers, especially when they're in the balloon (the wind rushing by), or on the beach (the sound of waves lapping on the sand). It's subtle, but adds more immersion to the film. And for you purists, there's always the original mono track.
The print is also very clean. Yes, there are some shots that don't fare as well as others, for various reasons - as mentioned in previously posted reviews, but I would venture to say around 90 to 95% of the film looks really fantastic.
Finally, I can watch Mysterious Island with a sense that it hasn't looked this good since its premiere in 1961 (and I doubt that it ever sounded this good).