The perspective can be breathtaking
I didn't like the lens on K10D, as it was difficult to focus it properly. On K-5 it is much better. It can focus as close as one inch from the front element, creating stunning perspective on close-up photos. No other wide angle I ever saw can do that.
Build quality is as good as it can be, as the lens is mostly made of some light alloy.
I only wish it had odd number of diaphragm blades and some kind of weather resistance. Then I'd give it 5 star rating without hesitation.