I think GIMP is fairly new on the market, but could be wrong. People don't buy photoshop generally, do they? Except for businesses? And even then, it is the name they are buying, not the product. If you said to a company, we should get GIMP, they would most likely be like, we have to pay for Photoshop, it must be better.
The thing I find with GIMP is that your desktop is your working space. I think that is something like macs, but I'm a windows boy, so like to have everything contained by the program... it is good though.
I reckon the biggest reason why I use ps over GIMP, is because I am used to PS, and know what I am doing. As a tool, I was really impressed by GIMP though.
http://www.gimp.org/