This article has some very kind words for OmniSketch.
This, hands down is my favorite app. It has 48 brushes – you get 24 and can buy 24 more. I has NO layers which is a downside and, while you can set the background color you can’t find white easily so you can’t set it to blue then go back to white easily. I know the developer realizes this is a problem so I’m hoping we’ll get a fix soon. I use this for drawing and for painting Seurat style images. I love its geometric and procedural brushes. It is a wonderful app and for 99cents it really rocks – it is way more deserving of attention than its current rating would suggest.