Keep in mind I'm upgrading from using a DVD player (Pioneer DV563) so all discs sound better.
Well-recorded discs like XRCDs sound much much better. As far as HDCDs, I only have a few but now will buy more as they are cheap. For the pop HDCDs (C. Wright, M. McBride, Dixie Chicks, etc.) there wasn't a big improvement but on the serious music (Lara St. John, Madeleine Peyroux, Reference Recordings reissues, etc.) astonishing depth was added. In fact, I'd say the Reference Recordings catalogue (http://referencerecordings.com/) is the biggest reason to own an HDCD player