From what I've heard, EOS bodies *don't* meter accurately with adapted lenses...
But since I'm (sigh!) considerting such a solution myself (current lack of alternatives), it might help if you said which kind of lenses you were interested in.
You mention Leica: Leica R or M? If R, which focal lenghts are you missing? (Pretty much the same goes for Nikon).
There are many good *obscure* lenses out there, but what do you need?