Saw this last night. If you're an Alien fan, you have to see this movie. Honestly, I hope they make another Alien movie soon too. Such a good series :D
It is a prequel to Alien, as for the 'stoopid greek aliens' it was referenced in the original alien movie as well. Everything lines up with the future movies aside from one single mess up.
Did anybody else notice all the Millenium references in it? They kept subtly referring to Lisbeth Salander. And then I realized that Noomi Rapace's character in this movie is Elizabeth.
Though the ending was left open, this prequel is not intended to be cannon. All reports are that the sequel to Prometheus will take the story even further from the original Alien.
I saw this yesterday with my lady friend and I have to say that it was a hit and miss for me. Parts of it were incredible and then other parts were sooooo dumb. Overall, I give it a 6/10.