I personally don't like Gimp, as the interface is kind of cluttered and the layout is much different to Photoshop (which is the industry standard). Also, the mod that changes the interface to look more like Photoshop is several version behind.
It's certainly not terrible for free software. It's just better suited to Linux, which is what it's intended for.
When I am away from my own computers, I use: http://www.sumopaint.com/home/ - Which is entirely online.
Well, that is your opinion. Im Vista user.
I personaly think the interface makes it only better as you can edit your workspace by puting together the Dialogs of each function you prefer for your work as you like. You could for example watch a movie while drawing on one screen at the same time ^^
Also a big plus is the commercial use in Gimp and it is always under develoopment.
I tried out that SumoPaint now and to be honest, in my case it got a big minus as it has no pen presure function. Even in pixel drawing for making sprites it has horrible quality...
The brush options are verry poor and making it online only can be difficult to use when you got no net. It doesnt has much functions compared to gimp. Also it doesnt look like you could use it for things like commissions. Just personal, non-commercial use.
Sorry if I sound kinda heavy but thats simply how i see it.