[e9999]

I would not mind having a bunch of low height action packers or similar up there for small stuff. Maybe later.
The most useful and safe use for me is all the bulky clothing and sleeping stuff. I use one of those inexpensive large soft plastic roof rack bags for luggage transport. I can put 4 or 5 duffel bags plus several sleeping bags and it still uses only about 1/4 of the roof rack.
I also put up my big firewood lugs up there and a couple of small tool boxes.

Note: I found that using a very long strap and wrapping things with it several times, going under the crossmembers each time is a great and flexible way to hold things up.