I don't take the case off very often, but I believe it's getting a bit scratched up, yes. The plastic itself won't scratch the case... the trouble is when the tiny bits of dirt start to get in, and the wheel rubs them around and around. You could probably protect the case with a layer of tape or something -- I haven't really looked into it.

Mine have moved pretty smoothly. I left the case slightly loose to help this. You basically need to fiddle with it and see where the issue is, but if you're seeing poor action, here's some things to consider:

1) Run a finger over the top surfaces of your prints. If you feel little prickly bits sticking up, you are probably extruding too much plastic and you might want to try increasing the filament diameter in your print settings. My filament measures right around 1.75mm, but I needed to set the filament diameter to 1.95 in order to get a relatively smooth top surface. You can use sandpaper or a razor blade to smooth things over a bit, too.

2) Is it binding mechanically? Maybe you need to trim the points of the star wheel slightly? I usually need to trim at least one, where it starts printing, since there's a little blob of plastic there. Also, some extra plastic squishes down on the first layer and leaves things a bit large.

3) Are you using ABS? Perhaps the parts shank/warped in a way I didn't anticipate. I've never printed this in ABS.

4) Is the stiff action around the ring that holds the moon wheel? Run a pencil around the outside of the moon wheel and the inside of the ring that holds it. Goal is to leave a bunch of powdered graphite on the plastic, to help lubricate things. I often find that mine is a bit stuff at first, but after using it a bit the action smooths out nicely. Not sure if the graphite actually helped in my case, or if it just needed to sit overnight and wear in a bit.

Glad you like it!
gwc made a derivative for the iphone4, which I hear is the same size as the 4s.
So you might try that: http://www.thingiverse.com/thi...