I can't comment on the questions concerning specific lenses that might cause problems or which Pentax cameras have trouble with the Pentax adapter - I have experience only with Takumar lenses, Helios, Pentacon, and a couple of other varieties.
However, here is the review I posted in the Misc Adapters section in the PF reviews:
I have had very good success with the genuine Pentax adapter, but have not used any of the 'after-market' brands.
The Pentax Mount Adapter K installs very smoothly; fits precisely, positively, and cleanly; and removes equally easily. It provides for convenient and trouble-free use in the field. It is machined to close tolerance, and mates well to the body mount. The threads provide for a close and wobble-free mounting of the lens. Needless to say, it allows the M42 lens to focus to infinity, which certain 'flanged' adapters do not. Finally, the adapter requires no modification for effective operation and no additional tool is required to dismount it - certain after-market varieties come with a small device that is used to dislocate the securing spring latch.
Concerning the availability and price: In 2011, I paid the full 'Canadian' retail price over the counter at a local dealer who had several on the shelf. I originally thought it was a steep price to pay for a small piece of metal; later, however, I appreciated its quality and ease of use and have not had any remorse. Recently, I bought another (second-hand) for a good price to provide more convenience and to serve as a back-up in case of loss or damage.
Highly recommended.
Craig