I think style and frequency of riding really determine what bag you need. For me, it's the Axiom Fondo H2O. It's not horribly huge (1L capacity,) but it holds tools, CO2, tube, bottle of Stan's, a couple of Clif bars (with room left over) and it has a neat part on the back that holds a water bottle. Some days, the ability to carry a third bottle is just a revelation. On those particularly hot days, I feel like that bag is the best thing I've ever purchased for a bike.

So... if you ride 20-30 miles twice a week, probably not your bag.