I have the late version of the Canon 35mm f2, as well as a 35mm Summicron v3. At some point I did a quick test comparing the two on a Sony nex.
Basically the Canon is sharper than the Summicron at f2 in the center only. The Summicron has more even sharpness into the corners. Stopping down, the Summicron overtakes the Canon even in the center. The corners do improve on the Canon the more you stop down, but they never catch up to the Leica. At f5.6 the Leica has very even high sharpness. I also looked at out of focus areas in the shots, but I honestly could not see any difference. Perhaps different subject matter would render differently than distant trees and buildings.
Compared to a lot of other lenses, even newer ones, the Canon holds up really well. Besides that it has great ergonomics, is light and compact. There really is a lot to like about it.
It is a very good lens, but don't expect miracles.