I have the Joe Rocket Manta tank bag. Fits just right, but it's only good enough for smallish stuff--a book and a shirt, maybe a water bottle if the book and shirt aren't too bulky. Sticks to the tank very well. There's a flap that is not deployed in this picture that reaches down to the seat, but it still fits. You have to take it off to put gas in your tank, but that's the same with most tank bags. It has a couple of pockets to fit a cell phone and extra stuff like keys or sunglasses or point-n-shoot camera. I can fit my work lunch in it.
