I've been an Acorn user for years now. I first started using it as my primary photo editing tool because I could open, edit, and export a perfectly-optimized web image before Photoshop had finished bouncing in the Dock. Photoshop has improved its launch time in recent versions, but Acorn has stepped up its game, too.
Acorn 5 came out this week, and it adds some powerful new features. Notably, it adds tools for vector manipulation and generation, as well as additional bezier and vector tools, PDF Import, snapping to grids, guides, other shapes, and more.
If you're a Photoshop user looking for an alternative, Acorn has the tools you're used to: dodge and burn, hue and curve adjustments, custom selection editing, and everything you need to do advanced photo editing. Acorn 5 can even import Photoshop brushes. Given the wide diversity of custom brushes available on the net, this opens up a lot of possibilities.
Version 5 also adds additional non-destructive filters and adjustments for both raster and vector layers, and the new Shape generators and processors are stackable and non-destructive as well. The layer adjustments are stored in the native Acorn file format, so you can always access and update them.
Acorn still has all of the great tools from version 4, including professional photo editing tools, Smart Layer Export for automatic 1x and 2x images, and the best compression on exported PNGs you're likely to find.
Acorn 5 is $24.99 US on the Mac App Store (also available for direct purchase, with a few small differences). Check out the website for more info, and read the release notes for a mind-boggling list of all of the new features.