This is a subject that has been debated back and forth in Porsche circles for years.
I've used them on many of my Porsches over the years. I've never noticed any difference in braking performance or feel.
It now seems undisputed among the most serious and experienced of Porsche mechanics (like Bruce Anderson, who used to work for Porsche's factory racing team) that in fact SS lines are *less* reliable than the factory rubber lines, for a variety of reasons. They are not as flexible, the joints can come undone easier, the metal can fray and puncture the line, and others. He reports seeing some failure of SS lines over the years, but says that it is almost unheard of for a factory line to fail. Even after 15 or 20 years. He thinks they are ok for a race car, which is inspected or taken apart after each race, but recommends OE lines for a street car.
As I said, I've used SS in the past with no problem, but over the years I've come to realize that OE is probably better on a street car. SS is kind of the answer to a question no one has asked: There is NO performance difference (if someone says they feel a difference, its probably because their braking system was flushed and bled after the installation), but less reliability. I personally am sticking with OE rubber lines in the future (which, in Porsche applications, are actually MORE expensive than the aftermarket SS lines).