FWIW, Joni's HDCD discs were at one time among the best sounding cds I owned(mid 90's), even without the HDCD decoding on playback. I remembered hearing a distinctive step up in sound quality with the HDCD disc vs the regular cds of the late '80s and '90s. They still sound great today. Currently many cds (in the genres I dwell) are done just as good or better as the technology has 'improved'.
I would get them if you like Joni Mitchell.