I have not used it for coffee yet, but have ground spices.  The first time, all the spice stayed inside the container.  The second time, I had some dust escape.  I do not know if I used the same grinding bowl and lid each time.  First grind was whole cloves, the second grind was onion powder that had clumped, so the escaping dust could have just been because it was onion powder which is already fine and gets everywhere.  My lids do "snap" on, but are a little loose.  It was not enough of a deal breaker, and did not ruin my day as some other reviews have indicated.  It cleaned up easily.  It did a quick job of grinding whole cloves into powder.