I have the BMW GSA specific tank bag. It's a nice piece and has lots of room. I like the easy pullback without disconnecting anything.
It's a little difficult to hook it, but you'll adapt a method that works best for you.
I was even considering adding a small pull to the plate to make it even easier. I can put my DSLR in it and still have
my clear visor at the top of the bag and not worry about damage. I have enough room for gloves and my GPS too.
It also has a removable pouch, but I just use it to store my visor cleaning stuff for easy access when fueling.
It's a little pricey, but all BMW tanks bags are. I like mine. I added some clear film over the paint to protect it from wear marking. It also helps protect it from dirt getting under it and scratching the paint.