No one has mentioned Tkx yet. I haven't tried it, so I can't comment.
I've experimented briefly with Prima, Wx and Gtk. The problem I have with these toolkits is that the docs are lacking. Prima and Wx seem particularly nice. AFAIK, Wx has the advantage of being the only gui toolkit that works on Mac OS X without X windows (except for CamelBones based apps, which only run on OSX).
I've done some big projects with pTk, and it definitely has some warts. But it wins by being well established and having lots of docs available, including a book.