I've one in my kit, and since I currently don't have a dedicated m42 body, it's used exclusively adapted on my Canon 10d. For the record, I've considered selling it a dozen times, but I rather like it.
For general use as a portrait lens, I find it's lacking in practice - I'm not familiar with alternate versions, but mine has a pre-set aperture, making field use just a tiny bit inconvenient. I've also noticed that results are somewhat low-contrast (if not hazy), and I don't think this is because of actual haze; my suspicion is that it's flare-sensitive.
That said, you'd wonder why I even like it.
What I
do use it for, actually, is still-life photography, usually with a mini-studio strobe setup. The ~135 mm equivalent on the 10d is just perfect for equipment photos in a light-tent, which I'll have to admit is somewhat of a fetish of mine. Most of the latter images
here were taken with this setup.
Still, I keep trying to make it a portrait shooter; I just like it. And it's also built like a tank. Hope this helps confuse your decision even more.
Chers,
--joe.