I purchased a set of HTR Z III tires from TireRack.com back in March 2010, had them installed on 4/25 2010 and have put on approximately 45,309miles on them, and still have about 3/32" +/- left on the tires, not bad for a 300 tread-wear tire. When I spoke to Tire Rack about getting new tires, they were shocked at the mileage, and noted that it was more than double the miles of the UTQG.
I have a 170miles R/T highway commute each day and seldom drive the car of the weekends. The car is a 2007 Subaru Impreza WRX STI and it is my daily driver/fun car. I don't drive aggressively, race or track the car. My average highway speed ranges between 75-85mph, sometimes more; rain/wet speed conditions 55-65mph, sometimes more depending; city speeds 35-50mph. I can't remember ever feeling unsafe with these tires in any conditions, maybe on very, very few occasions during serious downpours, but nothing that slowing down wouldn't fix. I have never had the opportunity to drive in the snow/light snow, so no comment there. I rotate the tires regularly in an X (front to rear/left to right) pattern at all my oil changes (sometimes between), and always maintain proper air pressure settings per the vehicle recommendations. I and don't believe that I've ever had to repair them to fix a leak, and actually one of the tires has the green goo in it for a small nail puncture, and still rides excellent. Overall these are an excellent tire for me, my car and my driving style under South FL conditions, and I'm very happy with them. Only negative thing I would say about them is that the noise level increases once you get into the higher mileage, at least for me.
Once again, I'm in the market for tires, and I think I've found the tire that works best for me and my car in the HTR Z III's, if something's not broken then why fix it, especially when it's such a great match between a car and a set of tires, not to mention the road.