You have to be very careful when screwing the lid down, as the plastic threads are not very tough and it's incredibly easy to cross thread it (which will make it leak and go everywhere when you shake it!) If you cross thread it too many times it will probably always leak. The product does not include a metal wire ball, which is standard for other brands to include and is really good for getting the protein broken up and dissolving quicker. I got a spare one from a friend and it made a huge difference, so I wish that it had been included as standard. The shape at the bottom also tends to leave a lot of protein powder stuck around the bottom edge, and if you have larger hands it will be really hard to clean! Adding the wire ball helps to reduce this problem, but not eliminate it entirely.
On the plus side, it is mostly waterproof, and the drinking cap on the lid seems fairly tough and unlikely to break. Not having a wire ball means you get to work harder shaking it up - and at the end of the day any extra exercise is good, right?