I have the large BMW Adventure bag on my GS. I really like it as I refuse to buy a tank bag that requires a separate rain cover. The BMW is waterproof and I did not have any seepage even in three days of Pacific Northwest rain. The unzip for fueling is kind of a pain but with the giant tank on the Adventure I don't find it that bad.
Best of luck as there are a lot of choices out there.
2012 GSA Triple Black