I have a roof bag, which I have used only a few times. I can't remember the make but I can look when I get home. I was impressed with it to be honest - it packs away to the size of, er, I dunno - two shoeboxes, a huge loaf of bread - not that big considering how much space is in it, anyway. Last time I used it, it absolutely leathered it down with rain for the four hour journey home and the contents were dry when we got home despite spending a few hours in a 70mph shower. It has as chunky zip with a velcro cover to go over the zip.
They are not as streamlined as a box - if you pack them with 'bag' they tend to get blown into odd shapes, but you can always just ram it full to get around this. Ours has a number of pole sections you fit together and then slide into pockets to give it rigidity - it does need roof bars to attach to, obviously - this it does with U bolts. I guess security wise it wouldn't be great if you had to stop off en-route and leave it in a hotel car park or something.
All in all I'm pretty pleased with it - I think I only paid £15 for it new. I do also have a 'tray' you fasten to roof bars to make an old fashioned roof-rack but I don't have roof bars foe the current car, so I don't use either currently - you're welcome to borrow the bag (although I might have trouble locating all the U bolts)