I love Qt, and much prefer working with it to Cocoa, but Qt on OS X is ugly and doesn't feel particularly native. I can usually recognize a Qt app as soon as it starts up. It still gets the job done in the cases where elegance doesn't matter (i.e. we have a tool for optimizing query parameters that has a Qt GUI that we just use internally), but for places where it does, it's better to go with Cocoa.