I've got an E460 top case and 2 B36N Business cases as side cases on my FJ and use it daily for commuting and the like. The E460 has never had any issues with water at all. Only time I've got water in the B36N cases is when I open them after sitting in the rain. The latch area forms a bit of a "well" when the cases are used as side cases that collected a small amount of water and I hadn't removed the case from the bike before opening it. That's the only complaint I can come up with regarding my Givi cases. As a reference for size, I've can fit both my helmet (Shoei X-11 XL) and my wife's helmet (Shoei X-11 M) into the E460 after I removed the "document holder" from the inside of the lid. I do end up laying them on thier sides to have them both fit, but that should give you a fairly good idea of the space inside the case.
The B36N Business Case is not as "thickly" built as the E460 and has more flex in the lid. Still doesn't leak, but if you try to cram stuff into it, you could probably make it leak far easier than the E460. And the B36N is really an oversized briefcase, best used to hold large, flat items. I wanted them for side cases that didn't make the bike too wide for daily use. The bike is still widest at the mirrors when I have the B36N cases on it!
Anyway... as top cases go, the Givi Vxx and Exxx series cases are hard to beat.