With the latest update, SketchBook Mobile 1.2 adds some new features, including tool opacity by tap-sliding the main interface element, the ‘puck’ (that element in the center):

Also, the top right icon let’s you go into the layer-drag&resize-mode, the coolest and unique feature for fingerpainting apps so far. The behaviour differs slightly from other tools: when you drag or resize, the changes are instantly applied. That means, when you, let’s say, blow up a layer, next try to scale it back down, everything out of the canvas-bounds will have been cut off. But once you get used to it, you can easily use the undos to get back to recent states. Another new feature: you can dublicate layers now.

The update is for free, the app is available for 2.39€/2.99$ in the app-store.