Probably the best art tool on mobile... ive been using the pro version, it came with my Note3 at launch, and havent found a better tool. 
The layering function is the best!
I love having a layer for line work, a layer for color, for shading, 1 for lighting and 1 for glow effects... maybe one for reflections and refractions...
its an awesome tool all artist should have. Especially if you're always on the go, its like taking a mini art studio with you wherever you go.