Acorn is the first of a new breed of lightweight Core Image based (i.e. GPU accelerated) image editing programs to arrive on the Mac to compete with Photoshop at the low end.
Acorn has a simple elegant interface and good, precise tools. It lacks some obvious features (custom brushes and erasers, rulers, guides, layer styles) compared with Photoshop, but it's more than enough for most non-professional users. On a well-specced machine the core image filters are incredibly fast and powerful, and Acorn wraps a pretty decent interface around them (more consistent than and superior in many respects to Photoshop's filter UI).