ArtRage, Painter and similar products are arguably a different category from programs like Photoshop, PaintShopPro and the Gimp in that the later are primarily designed for image manipulation where as the former are designed for drawing and painting on a computer.
My experience is mostly with Photoshop so I don't know Painter or any of the other digital art programs that well so I'm not sure what makes one better than another. My impression is ArtRage is extremely nice for $20. One feature ArtRage has that is probably standard in these types of packages but that I had not personally seen before is that when painting with oil the oil has a thinkness and dryness setting. If you paint thick non−dry oil and then draw over it with the palette knife or other utensil it will smear in proportion to it's thickness and dryness. Most programs I've used in the past didn't keep that kind of information. Another example, if you apply think oil and then draw a sharp pencil line through it you'll part the oil meaning you'll see the thinkness in the oil around the new pencil mark. Put different colors of paint down and smear them and they will mix into different colors. This also works for markers, draw with say a wet purple marker then draw over it with a yellow marker and it will smear and get mixed just like real markers.
Anyway, for $20 it's completely an impulse purchase. There is both a Windows and a OS X version and a free version as well.