I think so. Are there any big advantages? I would say that it has been good for me to take my own snaps with my Canon EOS 30D. I think Photoshop was too expensive so I tried GIMP. But it is also possible to have paid Photoshop licenses. For the free version I find it works OK, however some aspects would need further development.
And you mean for professional use, or would a novice find it possible to use in his amateur project?
My experience on GIMP
I have used GIMP for years on a Mac but I still say that for most basic imaging tasks PhotoShop is much better as it is less prone to bugs (although the latter is not true 😃). While I use GUI programs to do simple tasks, it is recommended to learn CLI tools for that, such as ImageMagick. I will post a tutorial on this soon.
For professional work (as distinct from the other stuff people do in here) you need a lot more than that. You need a solid command of your tools and the ability to make the compromises that will get the image where you want it. Also any kind of “posterization” style filters are of very limited use and don’t get the job done the way that PS or similar filters can.
A lot of GIMP users are in to very advanced stuff with lots of options in their tools but in my experience that’s not something that’s very necessary for doing the job you normally would do with a decent camera.
For an advanced amateur/moderner I think most people will find Photoshop too unwieldy and GIMP works OK, but it’s not a substitute.
If you want to use a camera, Gimp is good. But you can get better results if you have a camera that has a Raw capture mode.
Using camera Raw mode with GIMP
So if you use a camera with Raw capture mode you can go straight from camera to Gimp (or Photoshop) for editing. But I think it’s much more about learning how to use the software rather than learning what buttons to push or menu options.
With RAW capture mode you can make changes to the image much earlier than with other modes. So you can make adjustments to the image before you shoot it. Also if you need to make lots of adjustments you can save them as layers and then you don’t have to apply the changes again. (If you do a lot of adjusting and then get tired of the image just save as a new file.)
Another advantage of RAW capture mode is that you can do some processing of the image on your computer and then you don’t have to transfer it from camera to computer for processing. You can shoot from camera to computer for processing and then back to camera for output. If you make changes to the file in GIMP or Photoshop you can apply them from your camera directly (or through your computer, with a RAW capture file) to the same file.
If you really need a Photoshop-like interface, you can try PhotoGIMP plugin. It is open-source and free to use.
So, I think that GIMP is the best alternative to Photoshop, but a professional photographer has to know either PS or both.