Hi Bob - I used to have the 40mm S version when I had my SQ-b. It was great and excellently sharp (despite a small nick on the front element). I think the PS has better quality coating etc so should be even better.
the only drawback is if you use filters - a 92mm mount (I think) = v expensive filters. I ended up getting a Lee push-on holder for mine, which was still expensive.
It's a great lens for landscapes and odd perspective close - ups
these were all taken with it.