I sort of liked GTK back in the day when it was still the Gimp Tool Kit first and foremost. When it was 1999 and your other choices were a broken Lesstif, an early C++ centric Qt, clumsy Tk, and pre-Cambrian Xaw, it was nice to have something full-featured and tasteful.
Now I hesitate to pull in a GTK app because it won't theme right (I want to use the same bitmap fonts I liked in 1999, but apparently Pango stopped supporting them) and runs the risk of convincing the package manager to dump several gigs of GNOME crud on my drive.
I gather even the GIMP itself no longer tracks current GTK-- it's become solely in service to GNOME and their absurd UI whims (* * * * client side decorations)