Prometheus is a quasi-prequel to Alien. I say that because the press release claims it is not a prequel, but it sure seems like a prequel. At the very least, it takes place in the same universe as the Alien movies, so take that for what it's worth.
From the wikipedia synopsis:
In 2089, archaeologist couple Elizabeth Shaw and Charlie Holloway discover a star map among several unconnected ancient cultures. They interpret this as an invitation from humanity's forerunners, the "Engineers". Peter Weyland, the elderly founder of the Weyland Corporation, funds the creation of the scientific vessel Prometheus to follow the map to the distant moon LV-223. The ship's crew travels in stasis while the android David monitors their voyage. In 2093, the ship arrives, and its crew are informed of their mission to find the Engineers. Mission director Meredith Vickers orders them to avoid direct contact if the Engineers are found. The Prometheus lands near a structure and a team is sent to explore within.
Once the explore the structure, the movie becomes basically an Alien movie. There is life in the structure. Not exactly Aliens, but similar. Some people are killed, the android has secret motivations that put the crew's life in danger, stuff like that.
The movie looks incredible. It was filmed for 3D, so unlike many recent 3D movies, it actully uses the 3D really well. It isn't gimmicky, with things flying towards you. It just makes the scenery look more real.
I really liked the setup to this movie. In any good sci fi movie, it's thrilling to have that sense of going into the unknown. The crew is going to an alien world with no idea of what they will find. Unfortunately, there is a point in this movie where it starts to feel like a standard monster movie. It becomes, well, like an Alien movie. It is a very well made Alien movie and I wasn't bored, but I felt let down after the fantastic opening.
There is a really cool sequence late in the movie where a character has to perform surgery on herself. It is the coolest thing in the movie, and one of the most intense scenes I have seen in a long time.