The Cannondale Speedster seat bag is a deluxe rail-mounted bag that expands and has extra pockets, inside and out to organize your gear. Optimized design fits all your essentials; external pocket for mini tool or whatever you need quick access to.
Weaknesses: Stability, design, doesn't stay in place.
Strap doesn't stay in place. It slips over the front end of the bag allowing the bag to fall and rub on the back tire. I even stitched a peice of canvas to the underside to slip the strap through. It only worked a for a while. Poor design, junk.!!