Photoshop doesn't have a great UI, it was just very early, good enough, and targeted at professionals. GIMP is better in every way for me so far (I've just recently become a Photoshop user), except that CMYK doesn't work well or seamlessly yet on GIMP, making it unusable in a professional context. Other proprietary photo editors that worked with CMYK from their beginning are passing the GIMP as photoshop competitors.