I would really really like to hear from some owners that use a cargo bin on the roof for road trips in the 5. How bad does it hurt mileage, and how convenient is it?
I use a 18 cf cargo box with the OE roof rack. My last trip with this cargo box included 5 passngers and 4 bikes strung off the back of the car. I drove from Calgary, AB through the Rockies to Invermere, BC so you can bet that my mileage was not anything to write home about.
At the end of the trip I estimated 24mpg; which wasn't too bad when you factor in the weight and the type of driving.
As you've noted, there is no other alternative if you plan to use the '5' for long hauls. As for convenience, it's just like any other cargo box. And since the '5' is lower than a traditional minivan it's easier to access the cargo box.