The cup leaked as I shook up my shake the first time I used it. I've used it twice since thinking it was user error but each time I shake it, several drops of whatever liquid I am using come out of the bottle. As I drive around for work, I've resorted to putting it into a larger 40oz plastic cup to keep milk from getting inside my cooler. Even as the milk sloshes around as I drive (and even as the cup stays completely vertical) there is always milk that has leaked out. It keeps my drink cold, but leaking is inexcusable.