I recently put one on my Tamron 90mm after a frustrating time trying to clean the front element. It's so far recessed into the lens (why not just make the lens shorter? there can't be that much in the tube in front of the element surely?) that I slapped a UV filter on it. Not noticed any problems with image quality yet.
Tamron 90mm SP Di Macro
Tamron Adaptall 2 - 28mm, 28-80mm, 500mm Mirror
Pentax KAF - 18-55mm WR, 28-105mm PowerZoom
Pentax K - 50mm f/2, 50mm f/1.7, 28mm f/2.2