I purchased this eyepiece for an f6 telescope. What you get is excellent contrast and rich colors. The eyepiece was made for clusters like the Wild Duck. Planetary views show the subtlest details. Pentax, as usual, gets their coatings down. Less good is the edge definition. About 40% of the field is truly sharp.
I picked up one of these used, to give it a try. I allready have an older 12mm Type2 nagler, 12.4mm meade smoothside, and 12.5mm antares flat top orthoscopic in my collection. I found the comfort of the XF12 to be the best by a big margin. Suprisingly, despite more elements, the pentax was just as sharp and bright as either of the 2 planetary eyepieces, and noticeably brighter than the old nagler. The XF had slightly more scatter than the ortho, but much less than the nagler. It is a keeper! I will never be without a 12mm nagler (or possibly a 13mm ethos for deep space viewing with my Dob and TV101, but I will keep the XF for planetary and globular cluster viewing with my 8 ACF-R cassegrain and longer f8 4 refractor. A great EP at a great price!
See my thoughts for the 8.5mm.