I've never heard of any serious problems with it. Since it doesn't overwrite your standard firmware it's relatively safe - once you remove the battery and then reboot your camera is essentially back to factory settings.
I've tried it several times but don't use it regularly. It was pretty rough early on, but it's become much more refined & useful with the latest versions. My concern with using it is less to do with harm to the camera and more with harm to the footage - with some early versions I had it crash and lose the currently recording clip. I haven't seen that with the newer versions, but I also haven't had enough time to really test it in a non-critical setting to the point where I'm confident using it for anything important.