IINDIGO There are still a few UI inconsistencies (for example, selected menu items don't blink like normal Mac ones do - you click the item, the menu disappears) and I don't know if it's like this on an intel machine, but on my PPC iMac, 3.5 still has that "clunky" feeling where UI elements lag a second before redrawing. This causes it to still feel distinctly non-native.
Unfortunately, I don't believe this problem can be resolved unless Firefox does a Camino-style shedding of its XUL-powered UI.
(Version 3.5rc3)