Useful tool, but it has some bugs related to consecutive build failures. Restarting Air is required to fix it. I still use it, just wish I didn't have to restart it frequently.
Air also needs be restarted if you're switching git branches and there are a lot of changes between the branches
Shameless plug, but I built https://github.com/superhuman/lrt as simply as possible, explicitly to avoid this kind of issue (which seem to plague tools in this space).