I have 4 of these on different bikes. They will hold your water bottle, plus the bag is big enough to hold a frame pump, a spare tube, tools, and maybe even a small lunch.



Disadvantages: 1.) Don't work well on small frames, 2.) can interfere with rear brake cables a bit, and 3.) having your bottle up that high affects handling and acceleration some. I now know why frame builders always place water bottle bosses as low as possible on the frame. One way I have found to deal with this problem is to only carry empty bottles back there when possible, that helps a lot.