I've got a Rola that served me well for a while, but I eventually upgraded to a Thule box for a few reasons.
First, the bags all leak at the zipper, no way around it when you're driving 70 on the highway in a storm. Pretty much all the boxes will leak as well, but not nearly as bad. We had to double bag our clothes and sleeping bags if we thought it would rain with the bag. With the box we get a dribble but nothing severe.
Second, it's a pain when it's not full and even worse when empty. It'd go like this, get to camp, pull some stuff out, set up camp, then have to decide if we want to leave it flopping around half full or empty it and pull it off. And if you do empty it every time, you still have to pull it off.
Last, even when full it can flop around with a good wind. Since you have a full rack it won't damage your paint, but it's still annoying.
My suggestion is to get a box, but if money is tight I used our bag for a lot of trips before I upgraded.