Let me just say at the outset that this film was an outstanding success from my point of view. Speaking as an enormous fan of all the Alien films (even including Alien: Resurrection), Prometheus managed to do something I didn't think any film would in the series would be able to; it actually created the same emotional response in me that the original Alien film did. It was completely effective in evoking the sense of dread, anticipation, and if-only-they-knew-they-might-live-through-this angst that I felt when I saw Alien in the theater on a field trip to New York City (interestingly, since we had a few hours to kill before seeing the movie we were there to see-- the German version of Das Boot-- a friend and I took in Alien beforehand).
One piece of advice to you who might be fans of Alien; don't think of this as a "prequel". It is, and it isn't. If you go into this expecting to see face huggers and LV-426, you're going to be disappointed. It's... bigger than that. It doesn't set up Alien, it sets up the entire Alien universe. Go into this looking for broad themes, not specific artifacts.
Some reviewers thought the cast too large (large enough that the majority of the crew are just sort of extras waiting to die; the fact that this movie takes a heavy toll on the cast cannot, in any way, shape, matter, or form, be thought of as a spoiler-- it's setting up Alien, fer chrissakes...), but I thought it worked very well. Michael Fassbender (Magneto from X-Men: First Class) is characteristically brilliant as David, and he's written in such a way that the audience expectations regarding his character are well played upon.
It should go without saying that the visuals are flawless and stunning. They are all that and more. I could have done with a little less CGI and a little more mechanical effects, but I'm just old fashioned.
The music was interesting. There's a great motif that evokes a sense of exploration and wonder. However, it gets played throughout the film, but it's so short and is played so often that sometimes it really doesn't fit; people are dying, and they play the "wonder theme". On the other hand, there were some very nice nods to the musical motifs of the other films.
There were some nice dialog nods, too; not so heavy handed as, say, Star Wars, where the same line gets said three or four times across different movies, but they were definitely there ("we are LEAVING!").
On the whole, I thoroughly enjoyed this movie, and found it a worthy addition to the Alien canon. Leave your expectations and preconceptions behind, and fans of the original should get a real kick out of this film.