As with any bike accessory, seat bags have their positives and negatives. They add a little bit of weight, so you may not want to use one in a race, but for recreational riding, you can't beat the convenience of having your tools and spare tubes right there on your seat. The alternative can be a real pain–turning off your music, taking your headphones out, unclipping your waist belt and sternum strap, taking off your pack, and digging through 2000 cubic inches of stuff to find a tube. It's also nice to know that you definitely have a spare tube with you, regardless of whether you wore a pack or not.
The Fox Seat Bag is a pretty simple (and therefore lightweight) design. It's made with durable, water-resistant polyester fabric. The hook-and-loop straps make installation a snap. It comes in two sizes, and the large has an extra zipper around the bottom for easier access. The small will hold a spare tube, a multi-tool, some tire spoons, and a few other small essentials. The large holds all that plus another tube.
The Fox Seat Bag comes in two sizes–Small and Large–and is available in Black.