When I first considered this question, "how should future Star Wars fans first see the Saga?" I immediately went with my heart and gut.

Why it should be IV, V, VI, I, II, III of course!

At first I had no rationalization for this order other than it was how I saw them, and I just think the Original three films are better than the Prequels. Let me say here that I do love them all. But I have an affinity for the OT. I can't say there are any other movies that I love so much and devote so much time too. Now you would probably say, looking at my blog-"well you seem to write a heck of a lot more on the Prequels than the Originals". And I would say yes, that's right. Only because Episode III has just come out and has given me much to write about. As time goes by and I view the OT over, and over again, I am sure my Blog will steadily fill with OT themed entries...but I digress.

Having thought it out, I can see the positives for watching the Saga both in order, and not in order.
In my OPINION though (yes, this is just my OPINION), the positives for seeing the OT first far outweigh the postives for seeing the PT first.

Let me illustrate:

Watching the Saga in Release Order (IV, V, VI, I, II, III)
-You find out who Darth Vader is in relation to Luke (HUGE!)
-You find out Luke and Leia are twins (Huge!)
-You get to meet Yoda as an awesome puppet and he's much funnier
-You aren't bored to death in Episode IV by wondering where's Yoda? Where's Palpatine/Emperor?
-You aren't shocked at the sudden transition of FX, and the drastic change in style of lightsaber fighting. Instead you get to marvel at the steadily increasing talent at dueling in each movie.

Watching the Saga in Timeline Order I, II, III, IV, V, VI
-you witness the growth and evolution of Anakin Skywalker
-you don't really know that Palpatine is the "bad guy"
-you don't know if Yoda or Obi-Wan survive their respective battles
-you get flashy special effects and amazing lightsaber duels
-you see how Anakin becomes Darth Vader and then you continue as he seeks Luke then redeems himself (it really comes full circle).

These are only a few of the many reasons for watching the Saga one way or the other. Really it comes down to personal choice. Personally I had no problem knowing that Anakin became Darth Vader and still being interested in how he got that way. When you finish with Return of the Jedi you wonder, what made this man evil, and what confined him to the black suit? Here's where watching the Prequels after the Originals comes in. It's a little like the OT is Luke's story, and then we get to see Anakin's story and how it led to Luke's story. Though some would argue it's all Anakin's story-really, Vader has nothing much to do in IV except be menacing and torture Princess Leia, and note how "The Force is strong with this one". Having someone new to the saga watch the PT, then the OT...I could see how they may be bored. Because honestly IV is the one movie that doesn't really "fit". Which is why it must be viewed first. But I could go on forever. My point being...

In either case a person will lose some surprise or another. If you watch in release order, you know Obi-Wan and Yoda survive the Revenge of the Sith. If you watch in Timeline order, you know Darth Vader is Luke's father and that Luke and Leia are twins.
Well, in my mind the shock at learning who Vader is is MUCH bigger and more important then knowing if Obi-Wan or Yoda survive at the end of Revenge of the Sith.

But the key word in the original question is First. Watching it one way the first time doesn't mean people can't go back and experiment, watching it in different orders as they become more familiar with the Universe.
1st viewing, IV, V, VI, I, II, III
2nd viewing, I, II, III, IV, V, VI
3rd viewing IV, V, I, II, III, VI
4th viewing, I, II, IV, V, III, VI
and so on...

But when a new fan is being shown the Universe of Star Wars for the first time, I just think it has to be Star Wars that he/she see's.

But hey, it's only my opinion.