I have used Gimp for many years. It is actually the first full featured photo editor I had ever used. I had no idea about layers and paths and transparencies where I started. I found it very difficult to learn as documentation was sparse. Mostly though I use it for images that will go onto the web or into software applications. Most of the really technical features aren't required for that.
Now that I am very familiar with Gimp, I find it fantasic for photo editing and anything I can think of. Admittedly I'm an amateur and cannot think of much. I can't imagine a need for me to change to paid software now that I know how to use Gimp - bit again, maybe that is just the amateur talking.
As a computer geek, I also like being able to script for it, and if need be integrate its functionality into other programs. This is the joy of open source software. Free means more than just not paying for it.