I honestly don't see what the big deal is. Comparing GIMP usability to Photoshop usability only makes sense if you have no experience with either. Many people say GIMP is difficult yet forget they had a hard time learning Photoshop to begin with.
I personally find Photoshop daunting and GIMP pretty easy to use for most of what I need.
My dad is 70, likes to rant about broken/bad software and recently discovered GIMP for photo manipulation. He is very happy and has not said a single bad thing (which actually surprised me).
I never mentioned PS in my response, for me they are different and both of them are necessary.
In fact, from a year now, I always use Gimp as my image editing program after 4 years of PS, and I hope it doesnt die because it would leave me without choices...
This said, we cannot ignore the flaws on the UI, not comparing them to photoshop, but on their own. User life could be eased a lot fixing lots of little quirks the UI has.
Saying Gimp is pretty easy to use doesnt mean its UI could be improved a lot
I personally find Photoshop daunting and GIMP pretty easy to use for most of what I need.