Let me, as one who has never used Photoshop and am at best a very limited user of the GIMP offer one item unmentioned so far.
I am helping a friend build a web site. We needed to make three small graphics. On his Windows box we went to gimp.org, downloaded and installed, and in about 5 minutes had our graphics. Though I use GIMP on Linux, the interface was close enough that I could fire it up and go. We didn't trek to a store, pay $500 and then have a tool he would never use again that doubled the cost of his PC.
That goes without saying, you might say? Oh yeah, ask Gary which tool he prefers!
And I am not opposed to paying for software, and would gladly purchase the GIMP if available (in fact, I still have my CD I bought 6 years ago.)