You may have heard that there focusing discrepancies when using Jupiter lenses on a Leica.
These are very small, and when shooting in good light at f/8 or f/11 you are extremely unlikely to be able to see any lack of focusing accuracy.
Doing portrait at f/1.5 you may sell see a slight discrepancy,
check this link:
http://www.stevehuffphoto.com/2011/01/06/jupi-what-shooting-jupiter-lenses-on-the-leica-m9-by-robert-chisholm/