We're heading off on our road trip on Saturday and need to use a roof-top-carrier thingy for the first time (darned children growing and demanding legroom.....)
Friends have lend us a canvas strap-on (

), but there are hard boxes on sale at Sears this week, and we're bound to need one again in the future.
Does anybody have any experience of using these? Which would you recommend? Seems like the hard box may be easier to drive with (less resistance) and easier to attach, but will take up so much storage space and has no flexibility in it's dimensions.
All opinions appreciated, exspecially those packed with double entendres or creative ideas for child storage/size reduction