I used to work on a next-ish GUI toolkit (in plain C though, not Objective C) - http://runtimeterror.com/tech/lforms/ - which i mainly used under Window Maker (though it also has a Win32 backend) and my biggest issue UX-wise was how alien it felt in anything else outside Window Maker... especially the floating menus.
I like the look myself but i don't even use Window Maker itself that much nowadays (it is still my #1 WM for Linux, i just use Windows more).
I like the look myself but i don't even use Window Maker itself that much nowadays (it is still my #1 WM for Linux, i just use Windows more).