I have the new 450 husa-tm and it's already been dirty! Only problem so far is that the I need higher rate springs than standard as I'm over the stock weight range, but that's obviously not the bike's fault. Even so the suspension feels good, it's just sitting too low in the stroke at the moment but uprated springs will fix that and should make it soak up the bumps even better.
The engine feels really good, strong but very smooth and with a very linear throttle response. Maybe not quite as much punch as my 2010 450 but it only has 5 hours on it so I'd expect it to free up some yet, also my 2010 had the full factory exhaust which this doesn't (so far!) so hardly an equal comparison. I suspect it actually has more drive than you think, because of the linear delivery and maybe also because it's so quiet (no intake noise just in front of you and the stock pipe is quiet too). Whatever, I was never short of drive and it found traction well in some extremely muddy sections. Oddly, some of the reviews said this (and the 450 EXC) has very little engine braking, but I'd say it has just as much as I'd expect from a 450 (running 14/48 sprockets).
The most noticeable thing though is how much lighter it feels than the 2010, and how much more agile it is when you need to flick it about. The reviews certainly weren't wrong about that! Makes you wonder how incredible it would feel if the same technology was used in a 70 degree layout of course, but even with it upright it's a substantial improvement over the old engine so I'm not complaining.