Shoot, I was planning to post the same question, except with a more provocative title, like "What is the CORRECT order to watch Star Wars?"
I was actually thinking about what order to introduce them to somone (e.g., my kids) who hasn't seen them before. The big problem I see with watching them numerically is that, since you know Luke is Anakin's son, the impact of Vaders revelation to Luke at the end of V is lost or significantly reduced. On the other hand, if you watch in release order, then, as you said, you already know how the whole thing ends.