I love HDCDs, although I play them as FLAC rips. There is or was a list of HDCD releases somewhere online. It might still be there if you search for it.
Some years ago, I went through my entire CD collection to find all the HDCDs and rerip them to the higher bit depth. As you say, some of them were not even marked as HDCDs.
I found that most of them sounded pretty good and it was well worth the effort involved. Like you, I’m not sure if that was down to the format or if more care had been taken over the mastering.
I thought it was a shame the format didn’t catch on. I’d have been happy with them. As I saw it, the only real advantage that SACD, DVD Audio and Blu Ray Audio had over HDCD was surround capability.
They are not so common as new releases these days, although you can pick them up cheaply second hand. My latest find is Pieces by Manassas, which isn’t even a finished album but good nonetheless.
I understand that Microsoft effectively killed off the format when they bought the company behind it.

