Most here seem to fall in one camp or the other. There is the "it's always the Captains decision" camp and the "If I'm qualified to fly the plane I'm qualified to call a reject" camp.
It's not a black and white situation. It would be very nice if it was, if we could simply make a rule that applies in every single situation, but we can't, the situations are too varied and dynamic.
Blah Blah Blah's example where he had two events unfolding but the Captain was only aware of one is a good example. The Captain was forced to make a decision ("continue") based on half of the information. The fully qualified pilot sitting next to him was privvy to all of the information not because of experience, not because of how many stripes he has on his shoulders, but by virtue of the fact he was the PM at that moment.
If you reside in "Camp One" how do you think this situation should have been dealt with?
Incapacitation (especially subtle incapacitation) is another situation that highlights the fact that although it would be nice to put it in a box and say "it is always the Captains decision" , that is not always the best course of action.
So how do we stop 500hour F/O's from rejecting when the Captain has called 'continue' while at the same time recognising that in some rare situations this would indeed be the correct thing to do?
Ahhhhh aviation, gotta love it
