i am heavily using x11 under mac os x with ratpoison as my window manager. combined with mac's "spaces", this basically gives me full-screen x11 (but not actually full-screen with a root window) and a bunch of xterms in one screen, full-screen firefox in another, and then other mac applications in the rest.
i am also a heavy gimp user, but the attempts to port gimp to mac as a non-x11 app don't work very well at all. since i'm using fink, installing the regular x11 gimp was as easy as
fink install gimp2, but using gimp with ratpoison has never been fun.