That's certainly part of it in the professional world.
I've logged way more hours (as a hobbyist) in GIMP than I have in photoshop. I still dislike it mostly for the window management (which i've heard is better or at least changeable recently, but I haven't had a reason to go check) but a lot of other things (eg the file saving process) strike me as clumsy.
On a non-UI note, I wish it'd use multiple cores the way Lightroom (and I presume photoshop) happily will.