I\'ve noticed that GTK has a uniform feel across all platforms, which is great for Linux lovers, but not so great for people who prefer their native GUI feel.
I have
WxWidgets and Qt both look right on OS X, but don't work right unless you do a bit of extra work, because proper OS integration on OS X also requires that you get the basic AppleScript hooks right; OS X uses these for many common system GUI actions, so you need them to get (for example) drag-and-drop onto your dock icon working.