I installed Conti GP 4 Seasons 25c's exactly 13 months ago today, and they currently have 3,165 miles on them, almost all commuting miles. I've only had one flat, on the rear, about 2 months ago. It was a tiny hole in the tube and I couldn't see or feel anything in the tire. The rear tire is definitely showing signs of wear.

I love the tires, but not sure if I'll be able to go with another set. I got a super deal from a friend selling them brand new for $60/pair. Not sure if I'll be able to find them anywhere near that cheap again.