It does absolutely everything I used in Photoshop (Creative Suite 3) and costs a fraction of the price. Not to mention Pixelmator doesn't require a lot your computer's resources. Plus, I really like how it uses a ton of core OS X technologies, which really makes it the quickest way to edit, retouch and manipulate images on a Mac.

Friendly and familiar UI. Almost Photoshop-like. Pixelmator is a tool for image editing and graphics compositing - in addition to image enhancement you can use them to overlay and combine images into one, to add graphics and text, to turn photographs into graphics and make them look like fake line drawings or paintings. If you have Pixelmator for mac and want to work with image editing on the iPad I think Pixelmator for iPad is a must especially if you want to switch back and forth between Pixelmator on the Mac and the iPad. Because of iCloud you can easily continue working on your project whether it is on the iPad or on the Mac. Your project will be synced all te time. You can't do this with another app for iPad, unless you also have the mac version of this particular app.

The price. How can you beat it. It's magnitudes cheaper than Adobe Photoshop and basically does everything PS does. It's extremely easy to use as well.