We used ours many times when camping and, yes in heavy rain you can get damp inside. As we don't camp now they are still used, the last time being a trip to Ikea for a small wardrobe (just the rails as it wouldn't fit in the bag). A very handy piece of kit.
We've used one for the last four years, including in all day torrential rain driving back from Royan. We've had leaks and drips inside the car from the straps (on a Renault Scenic) but have never had anything inside the actual bag get wet. Just make sure you fold the cover down over the zip once it's on the roof and the contents will be fine, no need for double bagging.
Also, it's huge once you actually start putting stuff inside it! It can hold a lot of stuff, and because it's a bag not a solid box it's easier to squeeze stuff in as there's a bit of flexibility.
Make sure you follow the instructions for strapping it to the roof properly though...last year, despite having done it countless times before, OH managed to put it on backwards and it started to slip halfway down a French motorway. If you get used to the sounds it makes on your car, you'll know if something's wrong!
We've also invested in Handirack roofbars and a soft roofbag from Argos. Think we're going to put all our bedding/cushions/throws in vac bags and put them up there.
------------- May/June - Spring Valley
Aug/Sept - Leekworth