Just to mention that I am now using in my J3 a Sandisk microSDXC 64GB Ultra Class 6, bought it from B&H. I reformated it to FAT32 with gparted (I have a linux/gnome desktop, fairly recent install), and it works like a charm. Already filled up some 45GB in the card. The internal 32GB were already full. Some 13500 ogg files, working really well.
The card came with an adaptor and linux had no problems reading it.
The only glitch I've noticed is related to the UCI. I have a ton of classical music albums with very long "artist" names and browse my collection in library mode, by genre. Using the nice UCI's by kizune (Sense+Leaf) and I started to notice weirdnesses in the album list generated by the the Leaf browser with some albums showing out of order (e.g. one of my Rachmaninoff shows in-between two Bartok albums). I guess the problem is with the UCI.