I tried the Gimp out in October last year in relation to photography. I found it pretty horrible. I had to download a third party RAW plugin. The plugin didn't support one of my newer cameras RAW files. With the files it did support the UI was a mess, and vastly less intuitive then Adobes raw offering.

Once I had a photo in the Gimp, the most obvious thing missing was non destructive smart objects and filters/adjustment layers. These are a pretty big deal for me, which is a shame, because I would like to see the Gimp progress - it still has a long way to go.