If you've just switched to Mac and are looking for a replacement for MS Paint, then SketchBook Express is a good alternative. However, if you need something with a bit more power, then SketchBook Pro is what you need.
SketchBook Pro uses professional-grade tools that enable you to produce some highly impressive results. The streamlined interface allows for what is known as "gesture-based marking menus" which mean that different tools are available to you at the click of a mouse. However, note that to get the most out of SketchBook Pro, you'll need a pen and tablet device.
There are many preset tools, including pencils markers, airbrushes, erasers, flood fill tools, blur, and sharpeners plus different drawing modes that enable you to quickly create circles, lines and rectangles. There's also a quick cropping tool to trim your images.
Even though it uses the same pain engine, SketchBook Pro offers much more power and tools than SketchBook Express, including gesture support. However, many tablet users have reported that it can be just as awkward as the Express version at working with tablets properly.
Overall though, if you're looking for an professional sketching tool on Mac, SketchBook Pro is an impressive alternative.