I've been very happy with my 50mm "C" lens for my RB.
It gives me excellent, high resolution results both in the centre and at the edges.
It is a slight bit lower in contrast then my other lenses, but not in any problematic way.
It was very well used when I got it, and the shutter started acting up a bit, but after a CLA it is now working fine.
EDIT: I'm attaching a shot with the 50mm that originated on Ektachrome. What you see is the result of it being scanned at medium resolution, and then resized and sharpened a couple of times, but I think it might give a sense at how much fine detail the lens can handle.