Compared to the the collapsible and rigid Summicrons that I have used, the Canon 50/1.4 is very competitive. Similar in resolution and contrast to the rigid, IMO. At f2, the Summicron has a bit nicer OOF, but the Canon is faster and usually costs less. For "normal" situations, at f2 and above, I would have a very hard time telling the two lenses apart, even given a very large print.
The only reason to use one of the older Summicrons over the Canon is size, IMO.
More modern Summicrons (v3 and above) are a different story. Shorter min. focus, more contrast. More expensive. But then you might consider a Hexanon 50/2 instead.
Sean Reid has some tests that you could look at.
Roland.