You are right - The Rikenon XR is a more difficult lens to find, but well worth the effort IMHO.
I am not a Pentax snob by any means. My main lens is a Sigma 18-50 F2.8 EX. I use a Sigma 10-20 for a wideangle. For telephoto - I just bought a Pentax 60-250 after receiving three bad samples in a row of a Tamron 70-200 F2.8. I use a Pentax DA* 300 F4 for my long lens.
I just bought a Pentax K7 which I will be receiving this Tuesday. A good camera body can make all of my lenses look better.
I am a macro fanatic - I have a Vivitar Series 1 90/2.5, a Vivitar Series 1 105/2.5, a Pentax DFA 50/2.8 and a Pentax K 50/4. I need to pare this down at some point.
Originally Posted by Igilligan 
PentaxMan, I respectfully respect your right to 'First Right of Respectful Denial'
The Rikenon XR 135/2.8 is a fine lens and it has many of the qualities of the Vivitar and other leading leading 135's. But it is 'Rare as Hens Teeth' by comparison. Where as the Vivitar is so common that some people use them to tie down Helium balloons for the Forth of July Parade.
P.s. many as in most of the 135's have the built in Hood. The Vivitar and the Tair both have the built in hood...
It made me smile that I made a little dig at all the Pentax folk recommending Pentax lenses, and the first one to disagree with me is named PentaxMan and ironically recommending a Rikenon...
The rikenon is one of the better ones, and is a very good option as are many of the of the others mentioned in this thread. Other than the 50mm length, no other focal length has so many great old options...
With my tongue firmly planted in my cheek, and all of my respectful kidding aside...
Yours truly and correctly,
TairMan