I don’t normally write reviews unless I’m extremely disappointed.
However this is an App that merits some attention, not in a negative way. It does exactly what it’s made to do. It allows you the ability to straighten/alter the image without skewing the image and maintaining the 360° aspect ratio. It also allows you to recenter the image so you can choose the default initial view of the image when posting. It also allows you the ability of centering the Zenith or the Nadir in the middle of the picture for editing on other apps.
This is not a one button wonder that automatically does it for you. It is a manual app that requires a good eye and some skill. But with some practice, you can become very proficient at it.
Maybe at some point the developer may be able to create a way of automatically identifying the horizon line, but for now this is really the only option I know of that can do what it does on a mobile platform.