Anybody have one? I have lots of questions about how to use it, load it, what brand to buy, how big to go? Can you leave it on while empty (one brand says no)? How much noise does it make? Does it make the handling wierd? How badly does it affect fuel mileage and acceleration? Some of the bigger ones are pretty heavy when empty, how hard is it to mount the thing on the roof? Are they truly worthwhile? Hard to use? Would you buy one again? Just for starters.....any thoughts appreciated!