I haven't had my 450 very long, but I had my 426 for 2 years and probably rode it an average of once a week for that entire time. I'm not the fastest cat out there, far from it, but I never had a single problem with the Yammy that wasn't easily explained or self-induced. Had a loose plug once, failed to richen the pilot jet for a cold weather ride, etc. I changed the oil religously, as I do on my 450, and I never had to reshim the valves. To me, that speaks volumes of using a quality oil and changing it every 1-2 tanks of fuel.
The only thing I noticed about the 450 is that it likes quick, stabbing kicks to start as opposed to the 426 which liked the full follow-through kicks to light it off. I'll admit, until I figured this out, I was getting a little frustrated trying to light off the 450 a couple of times. All better now!