Ever since the Prequel Trilogy came out, Star Wars fans have disagreed over whether to watch the Star Wars saga in chronological order or the order of release. Although George Lucas prefers chronological order, both viewing orders have their pros and cons.
According to Lucas, the Star Wars saga is about Anakin Skywalker: his rise, fall, and redemption as a tragic hero. Watching the Prequels first makes this focus more clear. If you watch the Original Trilogy first, Darth Vader is a mysterious villain whose identity is only slowly revealed. If you watch the Prequel Trilogy first, on the other hand, you know who Darth Vader is and why; this makes it easier to view him as a sympathetic character.
Without the background of the Prequels, the central figure in the Original Trilogy is not Anakin, but Luke Skywalker. Watching the Original Trilogy first, therefore, makes the saga seem more like two separate stories: the Prequel Trilogy as the story of Vader's fall and the Original Trilogy as the story of Luke's quest to redeem him.
(George Lucas's intent brings up another possibility: one could choose to view the films in chronological order of production -- i.e., original trilogy first -- in order to assess how a filmmaker's approach to exposing a universe to his audiences changes over time and with the growth of a franchise.)
The viewing order is especially important if you're seeing Star Wars for the first time, because it affects how the plot is revealed. The famous plot twist in the Original Trilogy is, of course, "I am your father" (and, to a lesser extent, "Leia is my sister"). If you watch the Prequels first, this information is already known. The scene still has a lot of impact, however -- not from the surprise reveal, but from seeing how the characters react to it.
The two major plot twists in the Prequel Trilogy, on the other hand, are the identity of Darth Sidious and the fall of Darth Vader. Not only are these twists not shocking if you've seen the Original Trilogy first, but watching the prequel trilogy after the original trilogy makes the series conclude on a huge downer ending.
But the fact that the Prequel Trilogy was made after not only the Original Trilogy, but lots of other Star Wars media can change your perception and enjoyment of it. The Original Trilogy is fairly self-contained; most media that takes place between and before the films was published afterwards.
The Prequels, on the other hand, gloss over much of the background information about the setting and characters and contain larger time gaps between the films -- leaving the Expanded Universe to fill the gaps. This might leave you confused if you're not already familiar with the Star Wars universe. As a result, watching the Original Trilogy first could set a better stage for understanding the Prequel Trilogy.
The viewing order for the Star Wars films affects how the story is revealed. Will it irreparably shape your perception and enjoyment of the Star Wars universe? Probably not, as long as you keep the context in mind -- particularly the differences in setting and special effects technology. Regardless of how you watch them, your knowledge of each trilogy will deepen your understanding of the other.