Personnally i have no problem with GIMP interface, its pretty much like blender interface imo,
it may look a bit "WTF" when you first try it, then you set all windows like you want, become familiar with
keyboard shortcuts and you work 2-3 times faster than usual, with everything you need at hand =)
GIMP can also use photoshop brushes and photoshop plugins (with the pspi plugin), wich is a very big advantage
(Ive seen paint.net can also use photoshop brushes but it seems a bit buggy)
The only thing that piss me off in GIMP are these damn floating windows who doesnt reduce when you reduce the main window, i really hate that
Can't say about photoshop, never tried it >.<
PS. Gimp 2.8 will have a single window mode, and lots of new features
gimpusers.com/tutorials/gimp-2-8-features-preview-april-2010