Have the Wolfman Express Tank bag. Fairly small and suits me fine. Sturdy and easy access. Expandable. Easy to snap off and take with you if you parked away from where you are going. Has a pocket under the plastic map holder which is a handy place to stick any other documents you may need. Out of sight but easily accessable. Also has a zippered pocket in the inside of the lid.
Used recently on a 3 month trip in Mexico. Very usefull.
Bitches: Because it is squarish, did get in the way a bit when standing. I could have moved it forward a bit but was too lazy.
Great tank bag for me - would buy one again if needed.