I use a PS3 controller (bt) on Steam (linux) in Big Picture mode.
Ive been doing this for about 4 years or so & everything functions fine, EXCEPT....

....something that bugs me no end is that the on-screen keyboard within the Store interface just doesnt type any letters.

For example if you go to game > properties & enter a launch option, the keyboard opens up.
I can move around the letters, but nothing will type when i press the select (x) button. It clicks the letter, but no letter appears in the box.
Every other switch, button, slider, dropdown, etc works.