As far as 450s go.. In 14 I decided I wanted a new bike. I had only had Honda and Suzuki. So I wanted something different. I couldn't afford or find a deal on a Ktm or husky.. So that made the search for a yz or kx.. I ended up finding a deal on a 14 yz so I bought it.. My first impression was that it was the fastest 450 I've had (and I've had a fully built crf), specially bone stock.. I liked the ability to play with the bar mounts which I moved to the front holes with the clamps turned so it brought the bars back a hair.. Felt comfy, but the bike does have that wide feeling a little bit.. It was nice to have "decent" suspension. Bottoming was good, but it beat me up in the chop. I'm not an sss fanboy like a buddy of mine lol.. A revalve and springs may have cleared that up as I'm a little on the heavy side.. I had 2 issues.. I couldn't get a start on that thing. I think maybe cuz it hit so hard (which is weird, cuz I love a hard hit). Also, the front tire was climbing like Georgie has mentioned.. As soon as I junked the stock mx51f and put a mx32 on it, it was a totally different bike in corners.. I then trusted it.. It was also perfectly reliable. Never feared any type of failure.
However, I'd still like to own a kawi 450 once. This year I'll be trying the new Ktm 450..
Don't know if this helps, but that's my brief opinion of the yz. Overall great bike. If I planned to keep it longer I would have added a pipe, gearing change, suspension, and maybe play with the gytr tuner..