Flats: Having been in this business for a long time you just can't predict flats. What's more flats follow people more than they do gear. If you're the kind of person who gets flats then you're going to get them. More resistant gear will help reduce it but it's your riding style and choice of line and pressure, etc that are causing the flats. Personally I couldn't tell you the last time I got an honest to goodness flat from a puncture.
GP4000s II - Suuuuure. Everyone loves that tire. I have sold piles of them over the years. I completely agree with the sentiments above being that it is a Jack of many trades but master of none. It's a race worthy tire that can help against flats but rides like a concrete prophylactic.
GP5000 - Leah here at the shop (a cat 2 racer) has been riding one on the rear to replace the Pirelli she wore out. "I want to try them". She has 3000 miles on it already this year varying from training camp in Arizona to hours on rollers to races and rides all over Chicagoland and Wisconsin areas. Zero flats. Tire is fine.
Tires are tires. Conti was trying to pick up on a little of what they lost out on when they missed the boat by not making a tire that has a supple ride. I have no doubts that means the 5000 will be softer overall. That will lead to more of you on here reporting issues so this thread doesn't surprise me. That said it doesn't spoil the tire for me yet. I will continue to wait and see and in the meantime ride amazing tires like the Pirelli's