Maybe it's because I'm a software developer myself but I never had a problem with GIMPs UI. I never had a problem with Blenders UI either (only when they changed some things and I couldn't find them anymore because they are now somewhere else). In fact back in 2001 when I once tried Photoshop I found GIMP quicker to grasp than Photoshop (maybe because it has less features and thus one can get an overview of all the functions quicker).