I've been seriously trying out Sketchable in the recent weeks and noticed in the program credits a couple of names long associated with Painter: John Derry and Don Seegmiller. Lawrence Mann, a Corel Painter Master, is also associated with the product's UI design and sample art. Sketchable seems to be a spiritual descendent of Painter and you might want to give it a try.
It's certainly not a Painter replacement, but I can see it replacing the [increasingly annoying] Sketchbook Pro for sketching and no-fuss-get-in-and-draw situations. It's very fast and as natural media presets that aren't quite as overwhelming as Painter.