This is the daddy of seatpacks, with a capacity of five litres. It's as capacious as many bar bags and not far short of a saddlebag like Carradice's Barley. It dwarfs the 2-litre seatpack I usually use - big enough for tools, spare tube, energy bars, showerproof jacket and arm warmers.
I can't think of much that you'd need on a day ride, randonnee or audax that you couldn't fit into this seatpack. It fits to the seatpost with a couple of Velcro straps and to the saddle rails with two plastic buckle straps.
This isn't quite as convenient as designs such as those in the Topeak range which have a plastic clip under the saddle, although it does mean you can switch it between bikes easily. Despite its large size, it doesn't touch the back of your thighs. It's made from reasonably rainproof Cordura, although the constant spray from a mudguardless rear wheel on a wet road will eventually penetrate a bit.
There's a separate zipped side pocket, which is useful for keys or valuables, and a mesh pocket for a race cape or similar. A large reflective strip runs around the back, and there's a strap to attach an LED light.