I can't imagine what it would be like to watch the trilogy for the first time starting with the new movies first. I would definitely say watching the OT's first would be better, but I would be very interested to see how people reacted to seeing them with the new ones first. I think it would take alot away from the OT's; You woul know who Luke and Obi Wan really are, as well as C3PO and R2D2; And the whole scene introducing Yoda would be ruined; The "I am your father" scene would be a waste of time; The Emperor would be less interesting as we have already met him; Not sure how you would react to Boba when he first appears; and there are probably a few other things...

But like I said, it would be interesting to hear from the perspective of someone who watched the prequels first; did they find them believable? Did they seem to fit in with the OT's in terms of story or was it obvious that they were added as an afterthought? Questions noone who saw the OT's first can really answer without being biased.