Making prequels has become an art, and the successful ones are those that build a world which illuminate the original in a new light. That's exactly what Prometheus does to the 1979 Alien, ingeniously connecting loose ends 33 years later, and throwing in new philosophical questions about the origins of life and existence to ponder upon. There are a few wobbly narrative moments here, but the setting is so mesmerizing, the concepts so well-orchestrated, and the visual effects so enhancing, I'm waiting to see if any 2012 movie will impress me more. Highly demanding a sequel to this.
PS #1: I ran into a few great articles, such as here, here, and especially here. Also, there's a very interesting back story interview with Ridley Scott himself. Prometheus already seems to be positioning itself as one of cinema's sci-fi masterpieces.
PS #2: Ebert interestingly notes that compared to 2001: A Space Odyssey's opening sequence, Prometheus' jump from prehistoric times to the year 2089 has now grabbed the record for the longest-spanning flash forward in movie history. Cool.
PS #3: Thank you, Mohi, for recommending to watch the original Alien again before watching this movie. It was definitely a prerequisite, especially to see the "space jockey" scene again.