I believe they hold apprx 250lb. I read that in the owner's manual, I believe. I have a Thule Cargo box that I've used and it worked very well. If you think about it, 250lb. is about all you'll want up there anyway given the change you'll experience in center of gravity.
I use it for bikes. I have two Rocky mount bike racks attached directly to the factory rack. Works really well. I know the bikes don't weigh much, but there can be a decent amount of torque put on the racks when you go around a corner.
I have had no problems