HDCDs will play in any standard player.

I often wonder if the sound quality from these special formats is really due to the formats, or just special care taken during mastering/remastering.

I have a CD from Audiophile, has a piano recording done simultaneously through multiple formats. Analog, various tape speeds, different digital formats, some active filtering/shaping, and HDCD. The change among the various analog, and from analog to digital is pretty easy to pick out. I'd have a hard time being honest about being able to tell the difference between straight 16 bit/44 CD and HDCD.

So, if a disc has been remastered as part of going to HDCD, I would be suspect about any quality differences really resulting from HDCD itself.