Great tires, but beware the sidewalls
I had been riding my stock Espoir Sport tires (weighing 370 grams) on my Specialized Allez into the ground and it was time to start thinking about getting new treads. The GP 4000S tires were getting such fantastic reviews that I decided to go with them. They were a bit tough to mount on my Boyd Rouleur wheels--I had to use my tire bars to get them on. If you know what to do it's not that big a deal, but be ready.
These tires, at a couple hairs over 200 grams, are just over half the weight of my old ones. Where I really feel the difference is at speed--there are times when these tires just cut through the air like it was nothing. Just beautifully effortless. I hadn't experienced anything like that before. It's hard to believe that tires can so transform the bike's ride quality and performance, but these did.
As a bit of fair warning, though, the thin sidewalls of these tires can seriously offset the performance advantages. I rode one on just two rides and had a sidewall tear on the front. While I was going fast, fortunately it was in a straight line and I didn't crash. You may be lucky and not have this issue, but it certainly rattled my confidence in them.