The NWZ-A15 is certainly a good player - sound, operation, expandability of memory ...... everything okay. Compared to my old NWZ-S639F I see apart from the possibility of using a 128GB microSD card, no real progress. From the sound quality are both identical - who believes he gets the NWZ-A15 of the sound quality significantly noticeable update will be disappointed. High-Res Audio is a nice marketing gimmick. I hear from a 24bit song and an MP3 song (in 320kbps) no differences (have a few songs compared). High-Res Audio should be in my opinion no reason to buy. In addition, the NWZ-A15 in terms of maximum volume, much quieter than the NWZ-S639F. Volume "30 (max)" at A15 corresponds to approximately volume "23" when S639F ..... there is considerably more "air" to the top.
Otherwise you are fully acquainted with the operation and the sound settings, if you already own an older player from Sony. I guess the key-operated control and am glad that the A15 has no touch screen.
Since the memory expansion was the most important reason to buy for me, I'll keep the A15, although the S639F still works beautifully.
Conclusion: Who has still no player (and do not want to listen to music via mobile phone) can access safely. Those who already have a good older players should think carefully about whether a memory expansion + Bluetooth are worth the money. From high-res audio should not be seduced ....