I have used a FireFly stove on the AT many times; so did Sectionhiker on his recent long section hike. In a shelter, I would always do this outside the shelter (but I would do that with any stove). Because there are times when you may want an alternative, I would always have either a bit of Esbit or alcohol as a backup. These can also be used to help get wonky wood started if that's ever an issue. However, when you are burning just a bunch of little twigs, it's amazing how much useable fuel can be found in and around a shelter even on a rainy day.