Never used one, might if they weren't so expensive.
One thing that I think would be neat, that may very well be in existance already, is the option for sunroof access, i.e. a door in the floor. This would allow you to pass things up and down to the cab of the car as well as ingress and egress w/o being exposed to the elements during times of rain or snow(or when that bear is circling the truck

). This obviously would require a hatch in the roof rack and bottom of the tent, but I think might be an option worth persuing.
Ary
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HOKIES UNITED
Arya Ebrahimi
'94 FZJ80 215k miles, locked, 4.88s, 285 Revo's, ARB Bullbar, Hella 4000's, homemade drawers
'86 Toyota Pickup SAS'ed, welded, 4.88s, 38 TSLs, and lots o' tube