All of the functionality of The GIMP remains the same with the bonus of a much easier to use Photoshop style interface. One of the biggest complaints of people switching from Photoshop to The GIMP is the unintuitive GUI of it which is why the developer designed GIMPshop.
GIMPshop features menu layouts, dialogs and naming conventions that all Adobe Photoshop users will be familiar with. However, there are a few issues to be aware of when trying to install it on Mac. Firstly, you'll need Apple's X11 to run GIMPshop.
Secondly, the main problem that Leopard and Snow Leopard users will have is getting GIMPshop to remain stable. Although GIMPshop is supposed to officially support Leopard and Snow Leopard, the reality is quite different. However, it can easily be made to do so by modifying a library file. Instructions of how to do so are here and the libraries you need to make it work are here.
Of course, GIMPshop isn't a replacement for Photoshop but it's probably the closest thing you'll ever get for free.
- Version 2.2.11: Universal Binary