Roofline/Kammback transition - sharp or rounded?

[BrianAbington]

I've been wondering about this my self, I've been wondering how I can do something like this on my passat.

[Bicycle Bob]

That looks like the roof curve was getting ready for a nice extension, but had a visor added with just enough of a Gurney lip to make sure of a sharp transition and possibly create a stable attached vortex.