The first HT I built is doing fine after 2 years, any breakdowns I've had with it are mostly because I like to mess around with it to learn how they work. the only parts that I've had to replace so far from normal wear and tear is the tensioner and the clutch pads. In other repairs, due to because I like to mess around with it, I've had to replace the small gear (didn't put the retainer screw in tight enough, it came out, punched through the clutch cover and broke the gear), the cylinder (ported badly), the piston (chopped too much off that the exhaust and intake were open at the same time), the crank (drilled too much out of it and lost my primary compression). It's going on 2 years now
I have had bad experiences with the HT, but mostly good, the only other issue I've had with a couple of them is the gaskets on the magneto cover arent sealing properly, and its burning out the coil, (roommates chopper bike and my green bike, but 2 out of 11 isn't terribly bad), and the 415HD chain binds up in the sprocket cover/case, the kit chain is fine.