All you need, under your seat
This is a bag that does exactly what it is supposed to. It has a smooth shape that tucks neatly underneath the saddle, and I especially like the fact that it has a strap that connects to the seat post. It stays put, has a clean silhouette, and holds stuff. Not much more can be asked for from a saddle bag.
I have a medium and it comfortably holds a 700c tube, Pedro's tire levers, photo ID, cash, and a set of keys. If you plan on storing anything else (CO2 cartridge/chuck, cell phone, snacks) go with the large.