BMW OEM panniers are not sold as waterproof, the givi ones are.
I've had more than 1 set of OEM panniers, and when they are brand new, they usually do not leak.
However, the fasteners are riveted to the case. And after some use (and abuse), all the shacking and rocking of the bike will eventually make the rivets leak e little.
The topbox, with the fasteners at the bottom, seems to be less prone to leaking. Also, if you take a fall, the chances are that the case may be disformed more or less, also causing leaks. Hence the BMW solution for the strong and waterproof inner bag.
All in all I find the OEM cases of good quality and well built, but they are still limited to the law of physics.
I also have a Givi topbox. I find it of a lesser build quality than the BMW box. For instance, the top and bottom corners of the box, Givi cuts the plates straight, and the put on plastic pieces to make the corners, whereas BMW shapes the plates at the corners, weld them, and then puts a piece of plastic on the outside for good looks. Never the less, I'm happy with my Givi as well.
Also, when using an innerbag, the top edge on the BMW boxes is more or less flush with the sidepanels, making it easy to slide a fully loaded innerbag in and out, whereas the Givis (at least the one I have, Outback trekker) have a lip that protruding inwards, and this makes it a pain to pull up a loaded innerbag.
And the OEM boxes allow you to lock the box without needing a key every time you open it. This is very handy for the top-box. For the panniers, less so. And the key for the bike fits.
Still, Ii have several friends that are quite happy with their Givis, so I guess there is no right or wrong. It boils down to personal preference...
I know there are other brands as well, but I have no experience with them. But owners I have spoken to seems quite happy with their choice as well...
