You could try Patina, which is free in the Mac App Store. It's very polished and has high ratings.
Improved answer:
Patina has a very simple interface that makes the learning curve very short.
Patina gives the user a lot of control by allowing the user to turn on and off certain features, such as anti-aliasing and interpolation (for the latter one, not yet at the time of writing this but very soon and most likely already by the time most people are reading this post).
Other features include continuous object rotation (not limited to 45 or 90 degrees), transparent image saving, a slider for controlling tool width thickness, and alpha blending of colors.
