Stephan,
Just a minor note:
And I think we should address the performance hit. Although I suspect several, if not many, folks have ever noticed it.
I actually believe that almost all of the *felt* slowdown of rounded corners is fixed these days. It used to be the case that dragging an object with rounded corners around was significantly slower than dragging one without rounded corners (e.g., menus or windows) which makes the system feel a lot slower. This problem has been addressed (by a trick of yours truly, heh, heh ;) which makes dragging an object with rounded corners almost as fast as one without 'em (within a 10% range where it used to be a factor of two).
That's why I was asking for numbers in my previous postings ;)
Cheers, - Andreas