I traded in my old 500 IS for the new 500 II lens. My impressions after a month is only positive. This new 500 is lighter much more hand hold able for me. I am 60. Also at F4 and 5.6 with the older 1.4. It is incredibly sharp. It is really a great lens. I have taken a few dozen pic's with it now and posted if interested they are at my flickr site. www.flickr.com/photos/avianphotos
Oddly enough I have also kept my old sigma 500 and sigma 300 2.8 lenses in Pentax mount. Two great lenses IMO.