I watched this movie yesterday in IMAX 3D. Leave it to Ridley Scott to completely blow away any preconceptions I had about 3D being a worthless piece of technology. This had to be one of the more sumptuous movies I've ever seen.
As far as the movie itself is concerned, the first half of the movie definitely has the same DNA as Alien: long space trip, crew lands on an alien planet, crew explores weird alien artifacts, weird alien creatures appear but we do not understand how they function, slowly we realize that they are dangerous and predatory (although the audience knew this all along), the android ends up being a bad guy and wants to kill humans.
The difference is that while Alien kept the second half simple (Ripley's survival instincts kick in and all she is interested in is getting the hell out of dodge), Prometheus' second half is relatively complex and does not really answer any questions (why did the Engineers develop this weapon? Why did they create humans? Why are they trying to kill humans? Why are the aliens in this movie so different than the aliens in Alien?).
I'm not necessarily saying that I want answers to all those questions, since this allows people to discuss the movie and think about how it ties into the Alien movies, but it does not end on a satisfying note as does Alien.