Carried my 320lb DRZ 400S from Fresno to Moab last week, 865 miles, without any issues at all!
Used Ancra soft-ties and tie-downs at the fork, and a CanyonDancer on the bars as backup. Also a strap around the rear rim and wheel trough.
Solid as a rock (follow tightening instructions for the assembly bolts). I added a u-bolt type hitch stabilizer for a little more wobble prevention, but it certainly was not necessary.
I also have the lighter duty dirt bike version, but had bought it used, and the rail is too narrow to load my track bike (Duc Monster), which I'll be carting around in the spring. If I did it all over, I would have just gotten the Sport version with the higher load rating. It works perfectly with a dirt bike as well.