I think HDCD is a major improvement over standard redbook CD's. My own ears-on experience is also that HDCD discs typically sound better than their non-HDCD counterparts, even in a player without HDCD decoding. Comparing Japanese import Yes HDCD's against their U.S. remastered non-HDCD versions, the HDCD's were almost always more open sounding, with a much greater sense of space between instruments. I had two CD players, one with HDCD decoding, the other without. The better sound always traveled with the HDCD disc, regardless of which player it was in. I haven't determined the amount of improvement in an HDCD disc's playback with vs. without decoding, but when the HDCD LED lights up on my Muse Model 2 DAC, I know I'm in for a good show.
BTW, I can't compare to SACD; I've kept away because of the limited repertoire. Toto, Billy Joel, Cindy Lauper and a bunch of classical... no thanks. If Sony still had control of their Pink Floyd catalog, it might have been a different story.