This awesome mug is actually leak proof. AS in, fill it up, turn it upside down, and nothing comes out. There's a review of the pink, breast cancer research model on Amazon, and one of the three reviewers says that it leaks. If the cap is put on correctly, no, it doesn't.
This revolutionizes my working life. I made my first morning ride into work this a.m., all because my coffee was in my mug, strapped to my bike rack, and it was hot and all there when I arrived at the office.
I bought another manufacturer's leak-proof mug, which was not, not at all. But this one is absolutely water-tight. I bought it for $15; the other one was $25. (And will be returned.) Plus, it matches my bike. And the leather sleeve is not real leather. Score!