The crown jewel of Pentax lenses. This is as good as it gets folks.
First, the 31mm focal length on APS-C works out to 46mm, just wide of normal, and, coupled with f/1.8 maximum aperture, it's about as useful a lens as you can imagine for general shooting. Plus it's relatively compact, has a built in hood, and is one of the few autofocus lenses than can be enjoyably focused manually.
Build quality and handling are beyond reproach.
The image quality has to be taken in respect to it's design as a ultra-fast wide-angle full frame lens. Yes, wide open there is some softness, and you can find some purple fringing if you hunt... I won't deny this. Everything is extremely well controlled and corrected for, however, and stopped down it can pull in phenomenal frame-to-frame detail.
The FA31 is about several things that you will either "get" or you won't, and it's not really things that are measurable. First is the bokeh, quite simply amazing for a wide angle lens. Second the is the colors and contrast. The rendering of skin tones is exceptional. The brightness of bright light is vividly lifelike.
Not everyone can afford it. Not everyone will appreciate it. However, this is without doubt Pentax' finest.
There is a feeling of something being wasted on APS-C. It's a pity Pentax has no full frame digital SLR to allow the FA31 to be seen in it's full glory.