This fall, I bought the small version of the Cortech Dryver from Iron Pony in Columbus, Ohio. They made sure I got the correct BMW mount for the bag. The ring mount is a separate purchase from the bag.
Pros - it does not move around like other tank bags I have used.
it is a good size for items you want easy access to.
it has a small but usable map case that I use to supplement the GPS
Cons - it is kind of funky looking, sits high for a small bag
occasionally a pain to get remounted after fueling.
On the whole it serves the purpose and is reasonably water resistant.
Iron Pony normally has the BMW ring in stock.