I have seen
images taken with the Voigtländer 12mm Ultra Wide-Heliar Aspherical and they looked surprisingly good.
15mm sounds like the wide end of any cheap kit lens. 12mm sounds like something in the comfort zone of the ZD9-18, or the future M.ZD9-18, if not the Panasonic 7-14. Hmm.
In general adapted wide lenses cause some, or bad, corner/border smearing. Not all of the though. The 12mm Heliar seem to work fine. If the lens is the right lens is impossible to comment on. Manual, or automatic, zone focusing or the fiddling with the µ4/3 lenses. Second hand value? You've probably thought of al that. My choice, in the end, was to buy the ZD9-18 while waiting for a native lens covering the focal lengths.
Good luck,
/Jonas