Yes, and the 450. Look gorgeous when they're clean, look just like any other dirt bike when you ride them through a bog.
Moving back on the seat is odd, as the rear widens to accomodate the exhausts. Moving forwards is a doddle, front is very skinny. Suspension seems nice and balanced, the ones I tried were both set up for heavier riders than me, but weren't too harsh. Motor is very willing indeed, didn't have room to get any higher than third (short MX track) but flew out of corners like a two stroke. Almost an ideal comprimise between a two and a four stroke.
Note I said almost.
Finding grip on a slippery surface was a job for Rossi, not mere mortals. The engine is just a bit too willing, and the fuelling is instant. You think about twisting the grip, and the tyre lights up. 550 was marginally better than the 450, and a 450 with a single pipe was marginally better than that.
Two of my mates rode them in the last WOR, and they didn't enjoy the experience. In dry conditions they would be a great tool, don't know how reliable they will be long term though. I have heard that a different throttle cam makes a huge difference, but have yet to see the reality.
If you are really interested, I can get you up close and personal with one, but it will have to be mid to late February.
Mark