I took delivery of a used YZ450FX yesterday. 9.5 hours on it, the knobs on the back tire aren't even rounded. One radiator has been deformed a bit, though. Just like new otherwise. I haven't ridden it yet. It needs a bunch of set up before I'll be comfortable on it.
I've been riding a YZ250FX all summer. I love it in tight woods and when it is slippery. It is a bit under powered. Going from 13/51 to 13/53 made a big difference just about everywhere - lifting the front wheel on the trail, restarting a climb from a near dead stop, and makes second and third gear way more usable. I put a Shorai battery in it, removed the kick stand, removed the kick starter and installed aftermarket foot pegs. It isn't a featherweight, but it is an awesome handling bike. If only it had more power...
The 450FX has the exact same geometry as the 250FX. If I can get the 450 to handle nearly as good as the 250, I'll love it too.
The 250FX is my favorite woods bike thus far. A lot of people have said I should buy a YZ250X. The problem is that I haven't ridden one that has impressed me yet. I haven't ridden a 250X and all the YZ250s I've ridden have been poorly set up - suspension, powerband, jetting, etc. I ride with a guy who rides a YZ250. Great bike except his suspension is terrible and it is pipey. When it is slippery, he gets sideways a lot and crashes every time we ride in slippery conditions. He badly sprained his wrist on the first snow ride last fall. I did the same ride on my WR and had no issues.
I've ridden several KTM 350s. I see why people like them - nimble, light, etc. But it seems everyone who rides them hard has engine issues. My 250FX is way more stable than the 350s I've ridden. 300s for that matter too.
I'm not a racer so things like durability and cost mean a lot to me. I bought both my 250FX and 450FX used. Together I paid about 20% more than what a new KTM 350XCF would have cost. By the time you figure in that the KTM needs more maintenance and more suspension work, not to mention an expensive linkage guard, I could have 2 FXs for the cost of a single 350. It is a no brainer for me, especially because the wheels interchange on them.
Steering push and heavy steering are symptoms of too much sag. Forget about the tape measure, either decrease the sag or raise the forks in the clamps until it steers properly on the front wheel with no push.