none really. The only photoshop holdouts need CMYK or are unable to learn a new interface.
I only use photoshop because of all the free plugins that do what I want without having any skill at all. Butt hen I also think that my horribly out of date CS3 is just fine.
IF there were a lot more free plugins for GIMP that made it easier for us poseurs without any skill look good, it would surpass photoshop quickly.