The best tank bag in my opinion is the Givi 483. It is designed for the Versys, has a clear map pocket, is expandable and even has a waterproof cover. It was a bit awkward getting to the gas tank whenever I was fueling up but it was perfect for carrying rain gear, sunglasses, wallet, iPhone and iPad and water/road snacks etc.
I just got back from a 11,000 mile in two months trip around North America so I speak from experience.
"life is not a dress rehearsal"