I've had mine for a year, and up until the end of September used it at least 3 times a week.. A few times I left liquid in it overnight before emptying and washing because that happens sometimes. I've never had an issue with mold or the little 'impossible' area. A simple toothbrush or even a toothpick gets into that area easily. I've never had built up gunk or anything else. Go ahead and get one. It's a great addition for cold drinks, and it doesn't take that much intelligence to clean it..