GIMP does take some time to get used to, on a previous job version 2.6 was the only tool I had available when I needed to do some graphics work on the PCs. I was used to using photoshop and the Corel suite. So I sat down for a while and learned how to use it and then actually showed some colleagues who had attempted to use it before and given up and they actually became fans of it.
I do think the UI does require some work, this can often be a problem with open source software. The people who develop it aren't coming to use it for the first time so they find the UI easy where as newbies can struggle to work out how to do even basic functions. Such as have you ever tried to draw a filled shape in GIMP such as a circle or square? Not obvious how to do it for someone who has never used the software before.