I love that we can save money from buying K-cups by using bags of coffee with different flavors. I don't like the fact that I get coffee residue on the bottom and the top of my coffee cup. I've tried so many different coffee and I still get residue either floating and on bottom or just sitting at the bottom. I rarely use it now because that issue.
I've had the same issue. They make tiny little coffee filters you can put in there, though, and that seems to stop it. And they're usually compostable, so if you're using the reusable cup to cut down on waste, it still will!