I bought this so that I could take a second lens along with me when carrying my LowePro TopLoader Pro 75 AW, and it can be hung from the side of the aforementioned bag very securely. It does hang slightly lower than the end of the bag, but this is no real problem. It's made of water resistant fabric, and I think that it would need to get very wet for any water to drip inside, though it doesn't unfortunately have a built in waterproof hood.
It is ideal for the following lenses (and any shorter and narrower lenses):
Nikon 70-200mm f2.8 VR (lens hood reversed)
Nikon 24-70mm f2.8 (lens hood exended)
Nikon 105mm f2.8 VR (lens hood extended)
Solid performer, and only lack of waterproofing (though it is water resistant) makes it a 4/5.