With PhotoGIMP, Gimp is pretty usable, and I’d dare to say it can handle like 90% tasks (if not 100%) an average Joe has. If not for this beer weird interface, that would be a pretty usable piece of software. There are aspects that are much better than Photoshop.

For basic usage (crop, edit screenshots) I go for Pinta and can recommend. It’s fast, and usable too. No need to throw even more money into Adobe.