GIMP is widely reputated to be the most powerful free alternative to Photoshop. It is the defacto free photo editing software. For 3D enthusiasts, it can also be used to create texture maps as well as post-editing.
As a Photoshop and Photoplus user, I appreciate GIMP’s power. However, I also find the latter’s interface and quirks frustrating too. Therefore, there is a good reason why Photoshop remains the staple program to use amongst creative professionals. If you don’t mind paying for something cheaper than Photoshop and yet is still easy to use, you can look at alternatives such as Photoplus and Corel Paint Shop Pro.
As a workarounds to make GIMP more user friendly, I recommend using a Photoshop-friendly variant of GIMP. Alternatively, you can just get an affordable photoshop alternative with similar interface such as Photoplus.
There are 2 Photoshop-alike versions of GIMP, namely Gimpshop and Gimphoto. The advantage of using either Gimpshop or Gimphoto is that one do not have to mess with multiple-windows. You know how frustating it is to search for the right window.
With the photoshop-alikes, all the windows are contained in a single MDI.
Personally, I think for photo editing, Photoshop still pack more power. It is a great digital tool for a photography enthusiast. GIMP on the other hand is great for creating images for the Web. Nevertheless it still pack punch for photo editing.
That is just my opinion, though.