This is a truly wonderful lens. The IQ would be hard to beat in any way. The corner-to-corner sharpness is amazing. Look for the photozone review for the numbers on sharpness. The bokeh is lovely, creamy (9 aperture blades). The color rendition and contrast are excellent across the image. Very little distortion or CA (completely non-issues in practical terms.)
I've owned a long series of PENTAX primes over the last 30 years (50mm f/2, 50mm f/1.2, 28mm f/2.8, 40mm f/2.8, 85mm f/2, 135mm f/3.5, and others) not to mention some extremely sharp, recently produced zooms from other manufacturers. This is, by a fair margin, the sharpest lens I've owned. I've done direct tests against the PENTAX 50 f/1.2 and the PENTAX 85mm f/2, both of which I consider extremely sharp for film work (Kodachrome 64 and Tri-X-Pan, Plux-X, or Panatomic-X); and this lens blows them away in every aspect of IQ.
I wanted a fast, very sharp portrait lens. This lens fills the bill in spades. I also love how tiny it is: It takes up only a very small corner of my bag, and it's the smallest lens I currently own, so it is very unobtrusive and doesn't intimidate the subject. The 70mm focal length (35mm-equivalent of 105mm, generally considered the best portrait focal length) also gives it the reach to stay out of the subjects comfort zone. Wonderful for candids and portraits.
I chose this lens over the Pentax SMCP-FA 77mm f/1.8 Limited Lens with Case and Hood for the following reasons:
1. Better corner sharpness wide open (generally better edge sharpness, though a bit less at center though still excellent)
2. Less purple fringing
5. I don't really need f/1.8, especially in digital and the DOF at 2.4 is plenty shallow
I have only one negative with this lens, and it's a minor quibble: The minimum focus distance is rather long (27.6 inches according to the PENTAX website) but my intent was not macro anyway, no issue for me. I do intend to purchase the Pentax 100mm f/2.8 WR D FA smc Macro Lens for Pentax Digital SLR Cameras.
I was a little leery of the screw-in lens hood; but it works just fine. You can put filters under it or dispense with it altogether. I happen to really like it, both for flare and lens protection. The leather pouch is very nice. I keep coming back to the tiny size of this lens as well: A real plus in my opinion. Built like a tank: All metal.
I'm using this lens on a couple of K-X bodies. I love it!
Search the web for images made with this lens. The eye-candy abounds.
Super sharp, super IQ, a real bargain for the money. Enjoy.