but not bulletproof... i carried this full of water by hand the first time i used it and found a very very small leak forming on the bag when i hung it at basecamp. I must have cause it on a brier or something I don't know. That being said... the very slow drip stopped running and just forms a small bead of water on the surface of the bag that does not leak... so I can't be that upset being that it still performs well, however on the first trip and i made sure i kept my eye on it, it still got a microscopic puncture so i can't give it 5 stars. also, I love that my MSR Miniworks EX screws to it for contaminate free filtering and now i dont have to carry water in the bag anywhere when i dont need to! i can just get it after i find water from base camp.