Some people say a UI must be judged by experienced users, because newbies don't understand it yet. Others say a UI must be judged by newbies, because experienced users get desensitized to UI problems over time. (For example, every programmer can use their own program easily, but that doesn't mean the UI is good.) I think I trust the newbies more, because I've seen many bad UIs that experienced users like, but I haven't seen many bad UIs that newbies like.
This is not the only objective measure of UX.