I rode a few hundred kms in heavy rain with mine fully loaded and not a single drop inside.

 

The seal is pretty good and will handle most rain storms. I've just got back from a 1000miles around Scotland and not a single drop made it through. That said, I did get some seapage last year around Ireland. I always used a plastic bin liner to avoid disappointment, just in case.

 

I rode my '23GT+ 3100mi across the USA and encountered a handful of extreme storms (wind-driven, can't see where I'm going, just pull over with no shelter to wait it out pouring rain) and not a single drop got in. No leaks at all.