I just started on HDtracks the other day. I already have 10 albums. I used hd50get20 for 20% off $50 or more.
My impressions are things are just so much more detailed. You can hear more texture, separation, sound stage. Everything is just all around better. I am listening on Denon MM400's either directly from MBP>DF or iPhone 6 Plus>NuForce iCON iDO. Both are capable of 24/48, so I am trying to stick to that.
My favorite example so far is pretty much all of Alabama Shakes - Sound and Color. Just outstanding. I listen to a broad range of music and my second batch was a range of rap, country and rock. I was shocked on Kendrick Lamar - To Pimp a Butterfly, at the beginning of "Blacker the Berry" which I have played many times from the iTunes "mastered for iTunes" version. The 24 bit version has a woman talking in the background clear as day. Not sure if that was intentionally added to the 24 bit version or what the deal is. I need to go back to the "normal" version now to see if anything is audible and just not clear. Right now I'm listening to Brett Eldredge - Illinois which I have never heard before. So far I like the music (country) and the detail and separation is just phenomenal. Voices are so clear and you can hear the textures. I will now be going to HDtracks first for all my music purchases. And I'll thank my stars I bought the 128 gb iPhone!