I picked up a used 16 with the wide converter just to try it out...I had very low expectations based on reviews. I have to say though that it's my favorite lens now! So much fun and very sharp for most of the frame. I've been looking for cases where the poor corner sharpness has detracted, but I just have not seen it. If I do encounter that, I'll crop it out but when I am looking for IQ at the edges, stopping down to f/8 does the trick. I reviewed my past 10-15mm shots with a Sigma and Nikon 10-24 and those corners were not great either so it's no big deal - pretty typical for these kind of lenses.
The UWA adapter really makes the 16mm lens work for me...without it, I'd probably not carry it and just be happy with 18mm as my wide. But wow - the UWA is a real gem. I need to pick up a fisheye now - looks like another fun one!
I will post some pics later from my trip to Williamsburg, VA.