The 1:2.5 135mm is often criticized, in this thread the K is mentioned as a bad b*st*rd.
I have 2 of the even more maligned "Takumar Bayonet" versions and a Sears version.
I think the criticism may stem from users who don't understand the limited depth of field, so don't focus them properly in the correct plane.
here i did a test with the b*st*rd against the much praised SMC Pentax-M 1:1:4 100mm Macro , both at f/8 in deliberately difficult low tungsten light, manually focussed.
The DOF shows, along with my focus planes.
The 135mm
https://www.box.com/s/e8a00bcbf13764cb3da8
The 100mm
https://www.box.com/s/6fa59fddfd3cbbd1ea44
I will try some tests in bright sun tomorrow and wide open in the evening, also I have the much praised -M 1:2.8 100mm to compare.
I think the old Pentax 135 mm generally are sharp lenses and beautifully tight mechanically with metal body works, if users know how to use them.
Better shots, perhaps, than the average loosey goosey old nylon roller zoom with blurred corners etc.