I've had 2 different hard shell car top carriers that I've eventually sold off, because I end up using the soft shell most often in combination with my roof basket. It's just a lot easier to use, and much easier to store. They fold up to about the size of a shoebox when not in use, so storing it isn't a hassle.
Mine was also a generic one, worked fine, and only cost about $30. I don't use it for long periods of time, so I can't comment on how well it would hold up to prolonged sun or rain, but in normal rain my stuff stayed dry. If you don't plan on using it very often, I'd highly recommend this route. The only thing you have to watch for is to make sure your roof rack is compatible with the strapping system. Some of them require a certain type of roof bar to strap it down.
I should add that I don't use mine anymore due to a little run in with a low ceiling (you gotta be careful about that). I switched to using one or two really large duffel bags instead, each about half the size of the car top bag. It's a little more convienent because one person can take it off the car while full. Also, the stuff I usually carry never really filled the bag completely, so it never looked like a nice compact cube like in the ads, but more like a deflated balloon flapping around. I think they must stuff those things with cube ballons in the picture to make it looks so neat, none of my stuff was ever that square.