Prometheus was meant to fit into the Alien franchise as the film that answered all the questions posed by the original Alien movie. It does not and if you watch it expecting answers then you will be disappointed.
On its own, in isolation from the other movies in the Alien story, it's ok and worth watching. Don't expect much story telling though just enjoy the spectacle and feel the tension.
What it does do quite cleverly is fill a gap in the Alien story. It sits quite nicely at the beginning of the story where it doesn't have to do much in reality to start the ball rolling.
It's obvious that a follow on movie is required to further the story and answer the questions we all want answered, and hopefully they'll hire some good script writers and story writers for the next one.
So it's a bit like the coleslaw you get with your burger and chips. Doesn't actually add to the meal per se, but it fills the space on your plate making you think you got a bigger meal than you did, and is mildly tasty if you choose to eat it, but is otherwise entirely unnecessary. And when you're done, guess what, you still have room for pudding!
Hopefully pudding will be substantially more filling and leave us satisfied.