On edit: that jandd is really huge. Beats the Ortlieb, but it might be too big for some people

The biggest Ortlieb saddle bag is huge. I know a number of randonneurs that use it in lieu of a handlebar bag or a carradice saddlebag.

The closing mechanism is a little weird. It almost is an open bag with a strap across the back. Leave enough of the bag loose that you can overlap the sides of the bag. You twist one of the two straps up, and the other down. If you twist them both in the same direction, they can unwind and you will lose everything in the bag.