See I would say, if you haven't seen Alien. Don't see it before you see this. Definitely watch it after. Even if you didn't like the film. It's a must watch film.
I really enjoyed Prometheus. However I have noticed a lot of backlash is coming precisely because it is not Alien or Aliens. It's it's own completely different thing. Set in the same universe.
I think if people go, oh i'll see Alien. Which is an amazing film. Then if they go to Prometheus looking for more of that, they'll be disappointed. As it's a film about different things and really a completely different type of film.
Also it really is meant to stand on it's own. It's kind of like you go to the future and pluck different stories out to tll. Which may be connected but are different stories. I've seen so many misconceptions about the links. Prometheus for instance is set on LV-223. As opposed to Alien/Aliens on LV-426. People mistake this as being the same planet. As set up. When really it's not at all set up for Alien. It's a different look into the Aliens universe. Into elements of it. However it's going for a completely different angle, different kind of film.
I don't think this is spoiler alert, while Prometheus is obviously before Alien. The ship in Alien is older than the stuff in Prometheus.
Another interesting aspect is, the ambiguity to some aspects. A lot of people complain Prometheus doesn't spell everything out. However it's kind of central. The same way in Alien we have no idea where the ship came from, or the aliens. In Prometheus we get a more access to stuff, but really are none the wiser.
Anyway great sci-fi film. It also looks absolutely stunning.