Fans of previous Alien-franchise movies seem to be widely disappointed with this prequel, and I can make a guess why. It is not really a horror-suspense film like Alien, or a basically straight up action movie like (I gather) the near-equally acclaimed Aliens. In fact from what I can make out from vague memories of Alien (the only one I've seen) and recaps of other films, Prometheus has little to do with the franchise apart from being set it the same 'verse and featuring a Plucky Female Lead. Calling it a "prequel" may even be misleading because I am fairly certain the events depicted in Prometheus virtually guarantee a continuity screw-up somewhere, if only in terms of what characters ought to have known "later". So, for someone invested in the franchise, I can see lots of issues coming up.
Considered as a film standing on its own, though, it is a good one. The only major problem is the final scene, which is an unnecessary continuity nod. The visuals are superb; the scenario is intriguing; the conflicts are carefully set out; the resolution follows relatively well from the premises. On the downside, the movie is disjointed in places, and will likely not be winning any awards (especially with Avengers coming out this year, unless we still have awards people insist on only giving to "serious movies"). Apart from the leads much of the acting is adequate only. I would not be surprised to find out production was rushed: the end of the film relies more on impression than polish. It also feels rushed, and I cannot tell whether this was an artistic choice to communicate characters' emotions, or forced on the director to wrap up in time. If the former, it is less than entirely successful.
I consider my $11.50 well spent (and the extra four dollars to see the 3D likely would have been worth it), and would probably give the film an overall B or B-.
[Here There Be Spoilers]
The conflict in Prometheus really does not have anything to do with the aliens or the scientist's "Engineer" hypothesis. The movie, in my interpretation, really follows the different reactions of Dr. Shaw, human archeologist, and David, android linguistics expert, to the unfolding discoveries and then disaster. David, in fact, seems to sabotage the operation, whether from curiosity or malice - revenge? Shaw is an odd combination of curiosity, determination, and faith - seemingly unable to see past the questions she wants answered, but also capable enough to be the only survivor.
The unfolding horror element of the film is a scenario, a setting to watch reactions. A disturbing setting, which overshadows some of the more thoughtful elements and makes them lose their punch, unfortunately: fewer special effects and more "character time" would have improved the movie, if I am reading it correctly. The movie seems to set out to explore the question, "What does it mean to create, or be created?" - but even though the discoveries overwhelm the philosophy, I do not think the intended message is, "Let well enough alone," as attested by the end: continue the search.
The plot, here, concludes; but the questions remain. I find myself intrigued by this film, but unable to say exactly what my question is, even.