Eel - He doesn't have to wait until next year for a 450F! They're at my local shop now.... it'll just be a RED ONE not a Blue one!:p
mxbundy - Since I brought it up... you know the four stroke you are describing is the RED bike?( I'm not a RED guy or a Blue guy..I like'm all! LOL!) ;)
I'm debating a second YZ250P or a CRF to replace the 520. I've had a tough time going back and forth from two stroke to four.... the engine braking throws me off, until I adjust! (then it's time to swap and adjust again!). If you get back on a 2 stroke you'll have to relearn to use the rear brake and SHIFT! :)
I've raced D-37 desert on a YZ400F so I think I understand what you're asking. It's a trade off, the light weight of the two stroke is a VERY big plus to me. The YZ250 has a very smooth motor for a two stroke. I think the TPS and the ignition working together really help this bike be very smooth for a two stroke! I had an '00 CR-250 that I rode some off road, it was always a battle, the thing made power - but it was all or nothing! Not good for picking your way up a rocky hill !!!! The YZ is so different, it will lug, scream or just plain pull in the middle. So to answer your question, YES! IMHO you could ride this bike in long races without it being "tiring". But it's up to you and your riding style, the YZ250 will "roll on" nice and smooth but if you WHACK it open, it will try to stretch your arms! LOL!
I'm thinking hard about CRF, not that I think it's "better"...I just want to try the new toy too! :) And it's reported not to have much engine braking. Team Honda must think a lot of them as an "Off-road" bike, 3 out of 4 of their Off-road teams parked there 650's for it at the last "Best of the Desert" race. :)