I've played a handful of CDs back through my recently-acquired BDP-Pi and a LG blu-ray usb drive. I haven't picked up on any differences between the BDP-Pi and the Rega Apollo-R I normally use for CD playback (other than the Pi doesn't choke on the slighted disc imperfection). I listen mostly to rock (with is typically compressed) and jazz, not classical.
I've purchased or downloaded a few dozen high-resolution files and albums from Linn, HD Tracks, and a few others. Most I couldn't tell a difference, and the few that I did (only through headphones could I hear it), I could hear just the slightest difference for a few seconds here and there, but nothing that made it better. IMHO, well recorded and engineered CDs have fantastic sound and I don't sit around and wonder what may be missing from a high-resolution version other than $$ in my pocket. My caveat is I don't listen to classical music, but for the rock and jazz, I don't see how to justify it.
James recently posted a link to realhd-audio.com, which is an enlightening read on high resolution recording, mastering, and distribution.