Given the price, this lens really is great value for money. It has good corner to corner sharpness across the frame, even at lower apertures, and even wide open. Sharpness in the center is also good. As is customary for Pentax lenses, colour and contrast are also excellent. Bokeh, even of busy backgrounds, is perfectly acceptable, even wide open. I've not had the time to do a compare/contrast against other lenses, and besides which, I wouldn't be able to compare apples to apples anyway.
On the focusing side of things, with such a short throw, and light focusing mechanism, this lens focuses extremely fast on my K30, and doesn't seem to hunt much at all in low light. It also tracks fine as well. Quick-Shift focus is missing, which is a shame, but I don't think any manufacturer would offer this on this price class.
Build quality is also perfectly acceptable, and the plastic mount is not an issue to worry about in my usage scenarios (i.e. I tend to be careful with lenses, and not throw them on the floor - the only real scenario I can see where you'd have an issue - and that I'd expect with any lens!). The plastic construction also has advantages in that the lens is extremely light.
Length of the lens is approximately normal on the Pentax Crop Factor, and therefore has all the expected characteristics of a 50mm lens.
The only thing I would prefer is if it were half a stop faster, f2.0 or f1.8. But this isn't the end of the world with current ISO performance on a modern body.