Its an odd format, but its out there. Basically, if you play an HDCD with Windows Media Player (and only that player) you'll get even higher quality audio based on the data on the cd. If you use anything else, the extra resolution won't be played back. Sort of a catch-22, but there are ways around it if you want to dig into 3rd party stuff (I did and am glad I did so).
I only own one HDCD - the soundtrack to Princess Mononoke, and often times its tracks show up in the "Now Playing" topic. The HDCD version is about -8db off of clipping at 0db and seems to have a bit of filtering applied to the sound, but sounds amazing on good equipment. Without the HDCD info, the sound is pushed against the 0db brick wall and the audio clips badly at some points. There is definitely a noticeable difference in quality due to different mastering.
Too bad there aren't more classical/movie scores released in a higher quality format like this (despite jumping through hoops to get the hi-def rips).
