Adobe Photoshop vs GIMP for Serious Photographers
GIMP is poor man’s Photoshop. If we consider a simple task such as editing a digital photograph, it’s clear that Photoshop and GIMP can provide very similar working environments but is this free image editing software a good alternative to Photoshop CS3.
Compare Apple Aperture with Photoshop
For many people, GIMP offers all the image manipulation tools they need so ZDNet examines how far a serious (professional or semi-professional) user can get with GIMP vs Adobe Photoshop.
Although Photoshop just installs out of the box and is ready to go, to get advanced functionality out of GIMP requires some effort on the part of the user. Some standard Photoshop features are possible in GIMP, but require the installation of additional software.
If you use Photoshop to create artwork for print, then you can forget about replacing it with GIMP for now, as GIMP supports only RGB colour. CMYK support is due to be added, but for now it’s not available.
So can free software really compete with Photoshop? For the vast majority of ordinary users the short answer is certainly ‘yes’. However, for graphics professionals — that is, Photoshop’s target market — the answer has to be a resounding ‘no’. Link.

As someone who uses GIMP for photography, i would like to add some points.
There are 2 kinds of photographers who enhance the pictures out of the cam.
1. The perfectionist or artist who jazz up the pictures so they look like a perfect wallpaper, print professionals whose pics are to be printed in glossy magazines. This takes time, skill and taste. For this photoshop is the right tool.
2. The others like me just want to do couple basic things so the picture looks exactly the way as it should be. That consists of basic things like maximizing contrast, right colors and good sharpness. This requires hardly any time or skill. For such things GIMP is more than enough. Ofcoz gimp can do more but i am just talking about me who is a casual photographer who wants to fix photos which came out badly.
An example gallery of my usage of gimp is here: http://amitkulkarni.info/pics/singapore-bird-park/
The free book grokking the gimp is an excellent way to learn basics of GIMP. A photoshop user will learn GIMP through this book in no time http://gimp-savvy.com/BOOK/