I switched to the Pro3s a few years ago after running GP3000s and 4000s. I prefer the ride of the Pro3s, although their shelf life is shorter than the GP4000s. Like jmilliron, I'm just willing to replace them more often. Not bashing the Contis - just prefer the Pro3s.
On a side note, I've put some decent miles on the Pro4s I ordered from pbk. They're not cutting and pitting as easily as the Pro3s, but the ride quality is about the same. Since you can find the Pro3s now for about half the price of the Pro4s, I think that's the route to go.