Count me among the riders who have tried the GP4000's and a bunch of other similarly priced tires. My take (in chronological order)?
Michelin Krylion Carbon: long lasting, never seemed to flat (although one person's experience with flats on a bicycle tire really doesn't help figure out how durable they are!). Roll well, but not supple - the ride is kind of stiff legged. I liked the 25's much better. I got over 3,000 miles out of the back tires, close to 5k out of the front (and I'm on the large size - 220 pounds). On wet cold days, it was really easy to slide a tire, which is why I ultimately stopped using these.
Conti GP4000 and 4000s - as noted, these are the same tires if the 4000 are black. Ride noticeably better than the Michelins, even at similar pressures. Roll well (on a par with the Michelins) but with a more supple ride. I tried both the 23 and 25's - Conti tires seem to run more narrow than the Michelins. I preferred the 25s but it wasn't as cut and dried as it was with the Michelins. I got 2k miles out of the rear tire with these.
Vittoria Open Corsa CX - The best riding tires I've had on my bike. Supple, but still give you confidence when you dive into corners. Rated to silly levels of air pressure (140 psi), I ran them at 110f/115r. I used the 25c size. Would still be on my bike but they wore out very quickly - I was replacing the rear tire in not much more than 1200 miles. Like the Contis traction was fine - I never found the limits.
Michelin Pro Optimum - sold as a training tire, and only in a 25c size. The front tire is a softer compound, that will hopefully wear out at the same time as the rear. I now have 2000 miles on them and the rear is just starting to show signs of wear, so they are quite durable. They ride better than the GP4000s but nowhere near as well as the Open Corsas. So far for me, these have been the best compromise. Seem to work fine on cold, damp days - traction hasn't been an issue.
Tires are a set of compromises, so look for the compromises that work best for you and your riding.
JB