i picked up the a15 on a whim while in japan as it was cheap compared to home, until then my portable was usually my phone (initially a note 2 N7100 then a Oneplus One) both occasionally with my HRT microstreamer connected via OTG cables and used with my Etymotic ER4-PT with custom tips and Musical Fidelity MF100s (which were bought to tide me over while the cable was being replaced on my ER4s.)
ive never had the fiio so i cant really make a direct comparison there but as for the a15 i was pleasantly surprised, it comes reasonably close in clarity and detail to the HRT and thats no mean feat as i love the microstreamer, mostly ive used my own CD rips made with EAC (one day ill get dbpoweramp) all flac, i also have a few 24bit/96k albums too, sound quality is great all round even if there is only a little difference between the 24/96 and 16/44. ive even hooked it up to a bigger amp and speakers and it doesnt sound at all bad.
Pros:
The sound quality is excellent for the price
External SD card slot to swap out quickly and easily
Really light but excellent build quality
Nice controls and UI
Great battery life
Cons:
Proprietary cable rather than micro USB
Cant connect via USB to internal storage and SD card, its one or the other
Amplifier could be stronger
the only real problem ive got with it is the cable with everything else you know that even if you forget the charger you can always pick up a micro USB cable and youd be golden but the chances of finding a replacement cable in a pinch with this is annoying if you travel and forget the cable or something and although it provides lovely sound through my ER4Ps but with the S adapter on they are a bit quiet, however really aside from that the Pros far outweigh the Cons. of the other devices i tried with my headphones while i was looking this was the most impressive i heard for the money (there were no fiio players and the pono wasnt about then)
the other niggles are slight and im sure may be ironed out in future firmware releases, switching between internal storage and sd card is a bit of a clumsy process and its easier to just use a reader for the sd cards, on the plus side it worked perfectly under linux too, something that cant be said for most android phones (mtp and linux just dont play nice) and sometimes no matter what i do it just wont display album art for some albums no matter how many times i apply it, ive even used sonys own software and still nothing.
the problems are really just a few little niggles and sonically the A15 is a really nice sounding little device especially for the money, i really would like to try the fiio as ive always liked wolfson DACs generally but the A15 is an all round damn good little player for the price my only advice is dont turn on any of the "enhancing" features like "clearaudio" they just dont sound good