I really hate Solitary Shell in the context of the whole song of SDOIT, but it's a great 'song' on it's own, particularly in the way it tackles the theme of the song.
The bass kick bit from the end of GK going into the upbeat acoustic guitar is extremely jarring for me (and I'm a fan of jarring music), I really hate that. I think this is also because SS has no precedent in the Overture, whereas GK is a development of a harmonic progression from the Overture.
Yet the second part of SS is great in how it leads into ATC reprise.
My favorite portion of the song though is the first half though (Overture through to GK).
GK is a powerful movement, especially in context of the previous movements even though the lyrics aren't strong (despite the theme being traumatic and convicting).
In fact with GK and SS, it'd be great if the ending of GK went straight into 3:46 of SS, tbh.
(I also wrote some notes here:
https://www.dreamtheaterforums.org/boards/index.php?topic=56968.msg2828087#msg2828087)