This is possibly a must lens for all Pentax users. Why?
Well to start it is tiny. A true pancake lens, so a major talking point.
Does this mean it compromises in image quality? Not a bit of it. This lens is one of the sharpest lenses out there, a true addition to what Pentax are really good at, and that is small powerful primes that provide excellent image quality and rendition that in the real world give you shots that you go WOW!
When used on APS-C cameras, that angle of view is akin to a 60mm, but having said that, I have not come a problem with this. Remember, back in the 60's Nikon had a 58mm lens which was their "normal" prime at the time, so 60mm can and is a great FOV. Of course it is a 40mm and will act as such. Another bonus is that on fiilm cameras, this lens works excellently with a complete lack of vignetting. Give it a try.
Overall though, this is one wonderful lens and if you get one, you will have fun using it.
Not problems, but quirks.
The lens cap is screw on, but part of it is an inverted hood. In many ways genius! But I can some getting frustrated by the screw cap. I don't however.
It is "only" F2.8, but having said that, it is a pancake after all, and you can get great DOF control with wonderful Bokeh.
Like I say quirks and not problems.