I've heard less than stellar things about Prometheus (although Claudia Gray seems to like it), but I still wanted to see it. The trailers were fantastic and I love both Alien and Aliens. I haven't seen the other films in the series, due to that hearing less than stellar things thing.
I think Prometheus had one major problem: it's philosophy isn't that interesting. There is lots of listening to people talk about why humans were created and what their purpose is which is a question many people care about, but it's irrelevant to reality when the movie is proposing the answer lies with giants in space. Plus, the scientists have zero evidence for believing the aliens are benevolent creators, rather than people who just happened to visit Earth. And yet that plot takes up almost half of the movie.
Many people forget the pacing of Alien, how long it takes before a facehugger appears and then a guy gets his chest busted open. The fault in Prometheus is not that the aliens don't appear soon enough, but that what happens before the aliens appear is pseudoscientific nonsense and pseudophilosophical babble. And Prometheus does improve exponentially once the aliens appear and people start worrying more about survival than the origins of humanity.
The casting is fantastic. Noomi Rapace is beatific and miles away from her fierce, star-making role as Lisbeth Salander. Michael Fassbender is strangely inhuman and quietly furious at his own creators. Charlize Theron manages to make herself a plausible villain despite doing nothing truly villainous and being one of the most competent people in the movie. (Everyone in Prometheus makes at least one dumb, out of character mistake. Why is the geologist who maps the cave and can pinpoint his coordinates the one who gets lost?) And Idris Elba is fantastic as always and injects some real personality into the Prometheus's crew.
Prometheus could be much, much worse. But as sci-fi horror goes, it is no Alien. It does not live up to the promise of its trailer. However, I can fast forward through the worst babbling scenes to enjoy the true scares the second half of the film delivers.