Gimp is really one of the most incredible pieces of open-source/free software I have used.
But, it's primary advantages (open source, powerful) are what have led to its limited use. Like many open source projects, the interface is designed pragmatically, like emacs or vi, but this is a problem -- no professional has the time or the money to learn a complex UI.
The same goes for LibreOffice. Extremely powerful with a ton of untapped potential, but untapped nevertheless due to a dated and unintuitive UI. Inkscape and Scribus are better in this regard, and I've seen many professionals use them with outstanding results.
I've been able to create vectors with Inkscape without much difficulty. I can't say the same about Gimp. If someone really had the time and the dedication, they could re-engineer the entire frontend of Gimp from scratch for each platform, while leaving the excellent backend in place, but I doubt any sane individual would be able to do so without the help of the original developers due to the lack of full documentation available.
Still, it's worth trying. It's got 90% of the features of Ps but with 100% fewer dollars paid for it.