We have found these to be great a way to carry the large 1 litre MSR fuel bottles, which are normally too large to fit into a standard bottle cage. We have fitted ours under the down tube, so if there is any leakage it drips onto the road.
We have the MK3 version which has a quick release system that makes it easy to release from the bike but still gives a very secure fit onto the bike, even on really bumpy roads.
More info on the Bike Buddy web site.