Here I will cover a few issues I expect commentators to address, and I will then wrap up with my conclusions.
Colour
I realise that these test don't say anything about the colour rendering of this lens. But that is something I have never had any problems with. It renders like a Pentax should!
Crops
I am not presenting 100% crops as they basically don't matter. Only what you see here on the screen or in print matters. And I have made perfectly decent prints from images that look rather poor at 100%. The screen is already quite punishing compared to many prints. Of course if you need to print A1 or A0 size at full resolution, you will see more detail and more problems -- especially if that is all you are looking for. This depends on the distance you are from the print when viewing, lighting, paper, ink and other factors.
In short, this is a real world test. In the real world I present most of my photos as you see them here.
Smaller Apertures
I did not bother testing smaller apertures because I know from experience that this lens performs well at f/4 and so on. No-one pays the money for an extremely fast lens to use it specifically at a middling aperture. It is when the lens is close to wide open that we need to investigate performance.
Wide Open
This lens is usable at f/1.2. The biggest problem is getting focus, due to the extremely narrow depth of field and the fact that the body indicates that correct focus is achieved over a much wider range than is desirable. If you are far enough away from your subject you will get more DOF. If you are very good at manually focusing you will be a happier person. Learn to take numerous shots while focus bracketing.
The purple fringing at f/1.2 can be severe. For a mono image this is simply irrelevant. For colour, it will depend on your subject. Here is a front-lit subject with high contrast and fine detail with lots of white. I have not corrected for any fringing. Do you see a problem?
Schweppes at f/1.2 (processed) Bottom Line
Though you may have to take more care at f/1.2, stopped down to f/2 or more, this lens rocks. Of course there are other lenses that fast, but they in turn generally need to be stopped down one or two to get an optimal image.
Is the fastest of fast fifties worth the extra money? Only you can decide. But I hope this has given you some useful data points.