I love it! I just finished making a vid, so I'll up it tonight. A few ideas and issues.
-Text could use a bit more sprucing, though I like the font. If it were just a bit bolder and the lettering spaced out just a pinch I think it would be a lot easier to read. But it also sticks out of some menus, but not by much.
-The Crisis Core bgs have the title right over the crossbar, which is really distracting.
-Hopos are hardly noticeable at first. Increasing the "glowyness" should fix that.
-Starpower notes could use a bit more sprucing as well although those were much more noticable.
-I've always hated the sp neck overlay in Kid's theme. It's insanely distracting. I think you would be a lot better off with simply a green fade on either side with transparency in the middle.
-If you do use the above idea, change the sp notes to have a green tint, or just make them all a neon green (make sure it's a different green than the first fret).
-If you make the SP notes green, then you can make hopos white instead, which is a lot more noticable and may need little or no work.
I loved the sounds a lot, especially when you finish a song. Some of the bgs were distracting and clashed a bit, but in contrast there are some that really work well.
After watching the video, I have a few more ideas. Make the SP bar more like the limit break bar. Have it turn red or whatever the color is when you get enough to activate it. Or if you are going to use the green sp idea, you could make the SP bar green instead and apply the limit break idea to the multiplier.
x1 could be the color that you get when you are inflicted with sadness.
x2 could be normal.
x3 could be fury.
x4 could be limit break.
And it would turn neon green when you activated "Materia".
Video!
http://www.youtube.com/watch?v=aEsHY7qKZjg