I'm currently riding GP 4000 S II and I have no issues with these tires. They've worked well enough that I haven't felt the need to search for new tires. While shopping for new wheels and electing to purchase Zipp, I came across an article that stated the Zipp tires were the best match for Zipp wheels, essentially saying that they worked the best when paired together. However, I can't find any rolling resistance data, etc on them.

What are your experiences, what are your thoughts on these tires?