I mostly use Craft for syncing with Invision and the library to always have the last update for the colors and text styles of our style guide. It's really useful, and I can't complain except when it makes Sketch suddenly crash. At first, I thought this was a Sketch problem, but after looking into it, I learned that Craft could be sometimes instable. Still, it's worth the risk 😉
Am a recent convert to Sketch + Craft, and the connection to InVision raises productivity dramatically. Large files with lots of artboards/hotspots still face issues synching up, though, which is a bit annoying.
I use Principle mostly for quick demos when I try to communicate interaction ideas with my teammates. Principle is very good for this if you're not trying to create something too fancy but really want to share your ideas. Word to the makers: Keep the good work!