Further testing showed that the difference in image quality between the DA L 18-55mm and DA 18-135mm is not that large, and the image quality overall is in fact very good for a superzoom.
PopPhoto's SQF tests were more consistent with my experience with the DA 18-135mm, and the maximum magnification is in line with PopPhoto's tested value of 0.30x, rather than the 0.24x listed in the official specs.
To be honest, I normally trust Photozone's tests, but it's clear to me that this time, Photozone got a bad lens.
An example of what this lens can do:
Pentax K-5 with Pentax DA 18-135mm @ 100mm. M, 1/160s f/11 ISO 800. Bounce from shoe-mounted flash.
--DragonLord