I've received my Pono more than a month ago. Since then I've been using it with my two years old Shure 535 to play HD and Standard (Wav) files.
In a nutshell: Both standard and HD files sound great.
A couple of weeks ago I got stuck at Zurich airport, while there I had the opportunity to compare the Pono to the Sony NWZ 15B that the dealer happened to have on display.
I listened to Herbie's Watermelon man on both devices and Pono sounded more relaxed, open with a wider sound stage and details. But definitely the Sony sounded way better than Spotify on my iphone 5S.
Both Standard and HD albums sound great on Pono. I personally can't tell if HD files sound better than standard CD due to the hightest bit rate or better mastering of the albums but in most cases the difference can be heard, the same applies to my library of DSD albums.
Again for my ears some genres of music benefit more than others from higher grade players and speakers. Jazz and classical music lovers will be delighted by the Pono performance. Worth mentioning here that the Pono can really go loud, in fact Pono has enough power to drive my Sen HD650 to a decent volume.
Below is a list of tracks worth listening to on the Pono:
Hotel California - Hell Freezes over -Eagles (CD)
I heard it through the grapevine - Creedance Clearwater (HD 96khz/24bit)
Vivladi the four seasons, Violin Concertos-Elizabeth Blumenstock (CD)
Take Five -We're all together again for the 1st time- Dave Brubeck (CD)
In a sentimental mood -Art Blackey - Keystone3 (HD 88.2khz/ 24 bit)
Not one not two - One Guitar - Ottmar Liebert ( HD 96khz/24 bit)
Night train - Oscar Peterson (HD 96khz/24 bit)
Heart play -Charlie Haden (HD 96khz/24 bit)
La Scala- Keith Jarrett (CD)
The Endless River- Pink Floyd (CD)
Down the water line- Dire straits (HD 88.2khz/24 bit)
A saucerful of secrets- Pink Floyd (CD)