As someone who never used Photoshop before I find Gimp's UI functional and nothing worth complaining about. Especially I like the MDI interface with each widget being a seperate window.
It's been a long time, but when I was in high school I chose to use the GIMP rather than Photoshop for my photo class, which was the first time I used any photo editing software. I found it to be more or less fine.
(At that time, Photoshop was also MDI, IIRC.)