I've got the Hetre's on my 650B Kogswell. I've gotten a couple flats in a couple years of riding them, but they roll quick, are super at fast cornering and are damn good protection from flats.
I also rode 700C 29mm Cerfs for a little more than a year. They too are fantastic performers, but oh my! did I get flats on those. Riding in Baltimore was absolutely NOT COMPATIBLE with those tires. A year ago I rode a century ride south out of the city with those tires mounted. I managed to get home after repairing 5 (!) flats. Three were in the rear, two in the front. I found each and every piece as I repaired them- glass, wire and metal bits. I feared I would run out of patches or arm strength from pumping that day. I CANNOT recommend those tires unless you ride on very clean roads.
YMMV.. -Bob