If there is an M42 lens that is common, cheap, and has mediocre reputation, the Domiplan 50mm f2.8 is probably the prime candidate. You can still buy one for the cost of a lunch today. This East German lens has a few versions, and was available as Exakta or M42 mount. I personally have two versions of them: a pre-set aperture type, and a automatic aperture type. The one with the pre-set aperture does not work -- a very common aperture problem with this lens.
Doesn't matter from which angle you look at it, the Domiplan is a cheap lens. Designed to be cheap, made cheap, and feels cheap. Add to the fact that it does not have a manual aperture setting, using it on DSLRs is a challenge, unless your adapter was designed to handle this kind of lens. This is the reason my copy of the Domiplan was sitting on the shelf for a good five years, until I got a new M42 adapter with confirmation chip, which surprisingly, supports automatic aperture lenses.
[Update Oct-8-2010: The Domiplan is in fact a Triplet design. Thanks for all who pointed this out.]
The Domiplan has a Tessar design, which, in theory, should be pretty good optically. Invented almost a hundred years ago, the Tessar design is very common and was adapted by many manufacturers. The major advantage is the small size that can be made from this design. Most pancake lenses, such as the Pentax-M 40mm f2.8 and the Contax 45mm f2.8 employs the Tessar design. Zeiss refers to the Contax 45mm f2.8 pancake as the "eagle eye of your camera". Many just called it the "Eagle Eye", referring to its excellent sharpness.
The disadvantage of the Tessar design, is that it can't be made very fast. That's why most have a maximum aperture of f2.8. Another minor short coming is the minimum focus distance is relatively long, compared to other designs for the 50mm focal length. For the Contax 45mm f2.8, it's 0.6m, and the Domiplan is 0.7m.
OK, enough bubbling, I am sure you want to know how bad it is, no?
I didn't use the Domiplan on the 5D, so at least for the time being, we are not going to talk about the far edge performance, just the area covered by the 1.6x sensor of the Canon 550D (T2i). But even then, I can tell you that don't expect miracles at the corners, even stopped way down.
You maybe surprised, but I found the lens to be quite decent, again, if you don't need sharp edges. It has that Meyer-Optiks signature at wide apertures which I like a lot. Even at f2.8, it's more than usable at the center of the image. Here is the 100% crop of one taken at f2.8:
Wide Open @ f2.8, 100% crop at the center. Notice the slightly luminous, low contrast quality, but at the same time, very sharp and shows good details -- Canon Digital Rebel 550D & Meyer-Optik Domiplan 50mm f2.8. Larger Size.
Stopping down improves the sharpness as well as contrast. I would even rate this lens very sharp at the center. For the money of a lunch, it's not bad, eh?
The bokeh is a little wonky for my taste, but it's a subjective thing. I am sure some will find it pleasing. It's not horrible by any means. Just not great to my eyes.
Bokeh at f2.8 -- Canon Digital Rebel 550D & Meyer-Optik Domiplan 50mm f2.8.
So, I wouldn't mind using this lens, but certainly not for landscape work, where edge sharpness is desired. I find that despite its somewhat bad reputation that it has, it can indeed capture some nice pictures. My copy has very coarse focusing, probably due to the lubricant dried up, but for the money, I would not complain.
I love summer, oh wait, it's not summer yet! But who cares? - Canon 550D & Domiplan 50mm f2.8.
To conclude, if you overlook it's cheap looks, rough built quality, and it's blurry edge, and for how little you pay, it's a big bang for your buck!
Bang for your buck -- Digital Rebel 550D & Domiplan 50mm f2.8. Larger Size.