Aww, the indomitable 135mm focal length!
I own two different 135's. One is the Pentax-M 135mm f/3.5 and the other is the Sears 135mm f/2.8. I like both, but I really love the Sears. So much in fact that after my first one died with a broken aperture diaphragm, I'm now on my fourth one and this one so far holds up. This is maybe the one downside with this particular Sears: its aperture blades really have a proclivity towards drenching themselves in oil.
Since I shoot almost only film it is a true 135mm for me. It's on the short end of what I consider the perfect length for portraiture (135mm to 200mm). Whenever this comes up, it's the lens I use along with a yellow-green filter:
It's also a surprisingly sharp lens. I used it in some semi-macros situations (really more like a close-focus lens) with extension tubes:
Or this one (sharpness was less of a concern here):
And finally, but this is somewhat random, 135mm works very well when being on the edges of Manhattan or Brooklyn. Both the Hudson and the East River seem to be 135mm-friendly as you'll get just the right field of view when peeking across. :-)
Now, as for the reason why these lenses seem less popular on digital bodies: I suspect it's because a lot of them require stop-down metering. And indeed, I've barely used any of the above lenses with my K10D for that reason. At the equivalent of 200mm, you are already pretty long and will therefore have to work a little harder to get a sharp image. This additional step of pressing the green button always enraged me and would somehow manage to induce shake. Of course, all my film bodies have zero gripe with this as they will just meter wide-open and it's the ultimate convenience.
Cheers,
Tassilo