Fill tool completely jacked me up on this sketch. If you aren't doing your color on a separate layer than your outlines, you can pretty much forget about going back and fixing fill tool changes. It's amazing to be able to sketch on the phone, but if you think about the ratio of finger to screen, its like trying to use a baseball bat as the stylus on your wacom tablet.
That aside, some of the things I wanted to see in future versions of sketchbookX have been answered. Nice to see app developers responding to the community, even on a free app. Brush size on the eraser and visualizing brush size is extremely helpful!