Some time ago, my Photoshop died. More precisely, it failed to migrate and only exists on the old computer in the other room with the keyboard I don't like. So I have, after many years away, gone to try out GIMP again.
Gimp is a good program. It is genuinely full-featured image editing software. You can do just about anything you need to with it, once you find the right menu/tool.
Alas, Gimp is originally a Linux application and the 'port' for Mac operates in the X11 windowing system. I hate Mac+X11 with a great and mighty passion. It treats all the program palettes and toolbars as separate windows which you must first
click on to make active and then
select your tool, layer, etc. There is really no excuse for it to exist inside the Mac system except to allow lazy programmers to say that something is ported when, really, it's no such thing and a usability disaster to boot.
The Mac not-a-port does, however, have pleasantly native visuals and is not written in Java like some video players or open source office software I could name. It thereby escapes the half second lag that is the utmost limit of infuriating.
Gimp has a fairly short learning curve if you know Photoshop already, but there will be an adjustment period where you have to hunt for the right tool and sometimes look up ridiculously basic things on the web to figure out, for example, how to clear a selection to transparency. In some ways Gimp gives you more control; it makes fewer assumptions about what the user really wants to do than Photoshop. On the one hand, this means you have to do it all yourself, but on the other it means you have finer control of, for example, the cropping of layers to the visible canvas size. I do find the fact that it does not paste selections in as a new layer a bit trying, though, since that means you must always remember to create a blank layer to receive pastes so you can manipulate them fully.
History is a tab within the same module as Layers, which I find irritating given the click-and-click behavior required by X11, but, on the bright side, Control+Z can be used to back up as far as you like. I've never been a fan of the Mac behavior that Command+Z is both undo and redo for the last action.
Mac users will need to be aware that the Gimp 'port' does not allow use of the Mac keystroke commands. Instead of Command+S you will have to use Control+S. This is annoying, but the cmd vs ctl is the only difference--S will still save, Z will still undo--so it isn't more than an annoyance.
My one ongoing issue with Gimp is the file navigation, which sucks dead rat. Without ketchup. It does not provide for column-view or list-view of files, instead you have to double click through folder view after folder view to get where you're going. And it's ugly to boot.
The quality of the images produced is much of a muchness, though, and if you don't want to or can't shell out six hundred dollars for a new edition of Photoshop, you may want to download Gimp and give it a try.