Sketchbook is my favorite drawing app so far. The interface is excellent: It does get out of the way and makes excellent use of marking menus which allow super quick actions without the use of keyboard shortcuts just by penstrokes. Frequently needed actions like resizing the brush or pan/zoom have their own controls, also appearing right at your mouse or pen.
A rather mundane but great feature is that there is no proprietary save format. It saves in tiffs or psd.
I heard that the applications can be buggy. So far I met small problems like the marking menu not always working smoothly when using the mouse.
While a previous version was free, the price is still pretty good – the most relevant competitor with a simple interface, ArtRage, costs more; free/libré tools like MyPaint or Krita have more complex interfaces without the on-canvas controls.