Topeak Aero Medium Wedge Pack DX. The aerodynamic wedge shape lets these under seat bags slip easily through the wind while the internal pockets keep tools away from inner tubes and other delicate items. The DX medium bag provides just the right amount of
Strengths: It is bigger than it seems. The large compartment can carry tire tools, patch kit, a CO2 inflator plus a few spare cylinders, plus some hex wrenches and a palm screwdriver; in short, enough to get you through most scrapes. The smaller compartment is really just a zippered flat pocket, just right for a bike map.
The materials are well thought-out. The sides are made of heavy coated nylon, but the bottom and top are some kind of x laminated vinyl, kind of like truck tarpaulin material. It is more waterproof than the nylon, so it goes where rain and tire spray could hit it. The main compartment's zipper is water-resistant, of the coated inside-out style first used in ultralight rain jackets. (The other zipper is covered by a flap, so it is conventional.) Overall, it is not waterproof but close enough.
The main compartment's flap is hinged at the bottom, so that if you open it up, the stuff doesn't immediately spill out.
The bag clips into a bracket that attaches to your seat rails, so if you don't want to leave it on your bike, just undo the velcro strap and it slides right off.
Weaknesses: There are two weaknesses. One is installing the thing. The mounting bracket attaches to the seat rails with a kind of retaining bar and two screws. I had a hard time attaching the thing, apparently because it is designed for rails that are wider than in my seat (which is a Selle San Marco Ischia, hardly high end.) But that was a one-time issue. If you can't get it to work and are a bit handy, you could probably make your own retaining bar out of wood or aluminum, or skip it and attach the bracket with wire.
The other weakness, and this is a small one, is that when the bag is mounted, the rear panel is not vertical, but rather curves under, so that if you clip a light to the rear with the thoughtfully provided attachment loop, the light will be aimed about 45 degrees toward the ground. At least mine (a Planet Bike Superflash, highly recommended by the way) does. When I am bored I try to visualize brass sheeting folded in complicated ways to make a kind of adapter.