I am using some russian lenses (M42) with my 400D, and they perform OK. Also I modified one Tamron 200mm to M42 (and to canon via adapter), it was originally used on Minolta (
http://www.iceinspace.com.au/forum/s...t=minolta+lens).
Since I could not find appropriate Tamron Adapt_a_matic adapter, a friend of mine modified M42 screw from Hanimex 2.8/135 lens (it is a disaster of a lens, BTW, and I did not feel a thing when I canibalized it for parts) and machined it such that it fitted the lens, it was not very complicated job.
Of course, lens is manual now, but then again, I use it for astro use exclusively, so not an issue at all.
Generally, it is better to avoid adapters with lenses, they add a lot of distortions. It is better to consider mechanical modification, even if it is not reversible. After all, who wants to use lenses with film camera bodies anyway these days?
Only, you have to be sure the whole exercise is worth the effort. So some preliminary star testing with a simple adapter and eyepiece will be a good indication of weather to proceed with work or not.
Also, have a look at this link:
http://www.panoramaplanet.de/comp/
There are others.. however there was one very useful link that I can not find now.. I'll try later today from home.
Right now I am planning to modify Canon SSC 50mm F1.4 lens, FD mount.. it will be irreversible, but this lens is very good so it will be worth it (I even obtained the service manual for it!)