I use the Camelbak Flash-flo waistpack. I never even notice it's on, and it gets plenty hot here in the summer in NC. It doesn't hold as much water as the shoulder packs, but it's fine for the shorter rides I do. I find that the bottle gets too dirty on my mtb to be of any use anyway, so I never use one. If I were taking a really long ride (> 1.5 hours of riding), I would need to use a bottle along with the Flash-flo, though.