For complex creative work I use Gimp (2.8 Partha build), the free open-source powerhouse image editor that rivals Photoshop. But although I also have Pixelmator & Elements, for most routine jobs I rely on Acorn.
Acorn, Pixelmator & Elements have slightly different features, but they all perform similar non-destructive image editing functions, with many tools, layers, effects, blending & opacity controls, editable brushes & file support.
Elements is the oldest of the three, as its features show. Pixelmator has a trendy black interface, but Acorn's is simpler & more functional. They both use Apple's Core Image plugins, while Elements uses photoshop filters.
Functionally, I find Acorn to have the most useful tools & effects, including Quartz FX, plus the ability to combine them in uniquely different ways to produce an almost infinite variety of different results. Acorn can also use 3rd-party plugins like Java & Python scripts. Acorn's creator, Gus Meuller, often creates new Acorn plugins, and you can even make your own.
But a major factor is how they compare in terms of simplicity & speed. Acorn's simpler architecture not only makes it easier to use, it also makes it faster & more efficient; key features in any serious image editor, since imaging operations can involve many steps.
While Pixelmator isn't hard to use, Acorn often requires fewer steps. But it's not only faster, it's also more precise, since I've noticed that Pixelmator can blur edges slightly, often requiring extra work to fix. Also, I find Pixelmator's filter palette full of jiggling, animated tools to be silly & amateurish; appealing to children perhaps, but not useful in a serious image editor.
Of the three, Elements is the least efficient, hampered mainly by a clunky, antiquated color system, requiring multiple steps to choose colors. Acorn & Pixelmator both use Apple's great Color Picker, which is way simpler & more versatile than the Adobe system.
As you might expect, Pixelmator is the cheapest, at $29, making the aging Elements seem rather spendy at $99. But as the simplest, fastest, most useful & professional of the three, Acorn is easily the best deal at $49.