I still listen to one every single day. Love it. Perfectly portable, great external buttons and really decent sound quality.
Are the files 24/192? The J3 DAC chip can only decode 24/96 and will freeze if you try to play 24/192
a) its UI is as stable as it gets - that is, when you play the correct files on it
b) it plays fine my high bitrate Porcupine Tree flac files (which btw , as a band PT blow Muse out of the water - get some albums of them and see for yourself -no need to thank me always a pleasure to show music lovers something new)
I have a J3 since 2010 , no problems/complaints whatsoever - sure there are better (and more expensive) DAPs out there , but the combination of stable UI/value for money/easy to use/great battery/decent sound , i don't think there are many that surpass it.