Only people who were confused about what they were about see would say this was in any way a bad film.
Its not a direct prequel. It does lack the tension the Alien films are known for, but I felt it replaced that tension with more of a philoshophical approach to religion, faith, our origins and a brilliantly acted AI. It made me want more, it was a brilliantly stark, beautifully shot film. I wasnt confused by the plot as I didnt expect it to link to Alien at all and it didnt.
Maybe they should have steered away from ending the film as if Alien could be next, but I think that may well be the writer trying to fuck with our heads.
Loved it and I'm hoping the next film is something different again.
You must be the change you wish to see in the world - Ghandi