I have the Blowfish. It holds 100oz water, my pump, multi-tool, bandages, a towel, two spare tubes, contact lens drops, chapstick, an extra brake lever, seat collar, wallet, cell phone, keys. I always have room to spare. Most trail rides for me are (depending on time and conditions) 6mi - 30 mi.

A buddy of mine bought the same bag, but he packs his so full he actually has to use the expansion gizmo. Still, plenty of room, lots of water, not too much weight. If you go on longer rides, you may want to consider the M.U.L.E.