And the fact of the matter is that Photoshop has far from the world's greatest UI either (for one of the usual reasons--so much stuff has been incrementally added over time) so saying a UI is bad compared to Photoshop really is pretty damning. Contrast with Lightroom which, although I disagree with one or two of the fundamental design decisions, was coherently designed from the ground up.
The situation with The GIMP is really a lot like OpenOffice on a smaller scale. Lots of people would like a free image editing program but it's a big undertaking to produce something that's competitive with the proprietary alternatives and there's no commercial organization that sees benefit in supporting such an effort.
I bought Pixelmator years ago and it's great for what it is, but it's not really in the same class as Photoshop (or really even GIMP). The UI is clean and functional, but even as an occasional user I run into missing features.
I don't care if my photo editor is good enough for a pro. Pros use Photoshop. A good, free, lightweight editor is what most people need for removing red-eye, putting daemon horns on their third grade teacher, and cropping out their ex.
An open source editor will always be able to do high-end stuff, as long as it has some decent scripting capability. The rest of us just want something simple.
Take Open Office. The reason I switched was the better equation editor, and the tab-complete. I like FireFox because of the tabs and extensions.
There should be more focus on a minimal but usable product, and letting the extensions make up the feature gap.
A week away from GIMP to mess with Photoshop had me converted after the first couple of days, sadly. Photoshop is both a blessing and a curse for OSS.
(A blessing that it's a nice shiny target. A curse in that the target is half-way to the moon.)