I bought the large mug specifically for Rooibos teas or herbals where you can leave the leaves brewing for an extended period of time. I love that the filter strains the Rooibos when I am drinking it. I have never had any problem with the rooibos passing through the filter. I have tried brewing the tea the alternate way (by putting it in the filter and tipping it upside down after the required cooling period) and have gotten occasional leaks. I contacted the company and they have been very helpful with hints etc and even offered to send me a new seal! I am still not sold on the inverting method of brewing the tea in this mug—by the time the hot water cools to the point that i can invert it my black tea doesn’t seem to brew nicely; however it is great for brewing my rooibos. I use the mug everyday!