I did not set mine up properly for true deep litter; my pop door is too low. My litter is only about 4-5" deep and some does get kicked out into the run but that's no big deal since occasionally I throw some of the old out into the run before adding fresh. I also throw weeds in there as well as in the run, and give it an occasional shovel full of dirt. Before I did a complete bedding change it had a nice earthy smell that reminded me of a stable but without a manure smell... Sort of hard to explain. I loved it. My current bedding is 3 months in and has no smell. It's very dry even though it's been rainy and humid, so that is partly why there is no smell, but I'm thinking of watering it down a little to get some decomposition going. The downside to having it too dry, it seems, is that while you have dried out odorless poop it doesn't decompose as fast so it will build up more. I've never had an issue with mildew or mold. I think those would only be a problem if, for instance, your roof leaked or something. The poo doesn't include enough moisture for that (assuming your coop is sized properly), since bird's excretory systems are designed to conserve water.