Gimp is like the 'vi' of image editors. I hated it too but I slowly learned all the keys and now I find I can work at such incredible speed with Gimp that anything else seems awkward and slow. My right hand is on the mouse which never has to leave the canvas / work area and my left hand is dancing on the keyboard. Using Gimp primarily with a mouse is certainly a disaster of usability though.
Watch any photoshop expert and they all use hot keys as well so I don't think gIMP is any faster in that area.
But otherwise I think it really depends on the features you use. I use Photoshop's layer effects all the time. Anytime I want to point something out on a map or a screenshot I drop it in photoshop, draw an arrow, add some text. Then set a layer effect that makes the arrow and text a bright color with a black outline and a drop shadow. If I need to add more arrows or more text they get automatically styled the same.
AFAIK that's not simple in gIMP or even available.
Similarly I use photoshop's non-destructive layer adjustments all the time. I don't know how I'd get by without them.
My understanding is gIMP will be adding those some day. 3.0 maybe?
