It's like an attic for your car
I've been using this box for a few years now. I figure it has about 25K miles on it by now. From Florida to Washington and everywhere in between. I've crammed enough for a family of 3 plus my 2 70lb dogs toys and food. We put a lot of miles on the road with camping and road trips and this thing has never disappointed.
I have this on my Honda Element on factory aero bars and have really put it through some testing. It seems to perform great in windy conditions and I for sure don't take it easy on the gas pedal when we're on the highway. I think it's safe to say you can clamp it down and forget about it.
Some things that might be interesting: Skis up to 182 fit without any problem for me in addition to 3 other snowboards. I think 1 pair of skis and 3 snow boards are probably just about maxed out but I think another pair of skis could be fit if you are a Tetris rock star (which I am, of course)
I'm not incredibly tall, strong or clever and I seem to have no problem putting this on the top of my Elemen without any help, so if you're concerned about it being a pain in the butt to get on and off, it's definitely not.