pretty good, I like the feel of it. I have one problem and one suggestion though. When you run on the top off the circle you sort of run then fall then run... This goes along with my suggestion,I think you should have him orient himself so he's standing perpendicular to the ground.
Using this you could add in the ability to run upside down.(around the circle for instance). If you would need any help doing this then I would have no problem giving you some pseudocode/helping.
That would require a pretty complex physics engine. It's possible, but it's certainly more complex than just using slope detection to align the gravity.

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