I use Gimp because it's open source. There's an entire community of users helping to make it better and improve what's already there. I don't feel like I'm being forced into doing or buy something because of some "pencil necked geek" in a corporate building somewhere says so.
I have both Gimp and CS5, but I didn't buy Gimp or the copy of CS5. I've always used Gimp, and rarely have a need to use my CS5.
Photoshop wins the feature war hands down, (if it can be called a war). It is an older, more mature project with a huge staff of very capable programmers. That said, most users need only a tiny fraction of what the program has to offer. If you want to create a great-looking logo or create some icons or dress up a web page, Gimp is more than up to the task.
I haven't need to us PS for anything in a very long time and currently it's uninstalled.
I did plop down the $70 or so dollars a few years ago for PS Elements 8, and in retrospect it was a wast of money. It too is uninstalled.
My Gimp is something I'm actually proud of. Part of the fun was creating a sort of personality for my Gimp.
Being able to add or remove things as I see fit. I would be very unhappy to not have the Gimp I have.
To me PS seems kind of sterile and flat.
I'll always be Gimping, but I can't say that for PS.