Overall I think a B or B+ is a fair grade for Prometheus. I feel like the story is being maligned a bit unfairly. The optimism and adventure of exploration is not a very popular topic to tackle, especially on the big screen, and the film kind of had to mesh that quality of science fiction effectively with its franchise reputation/DNA for suspense and horror. The downside - as I think others have touched on - is that it ends up doing neither very well. Still, I appreciated the effort very much and the overall mix is at least entertaining to watch.
I think the best part of the film
On the other hand, I think I'm also just pumped to see decent sci-fi again, and to see the theaters full of interested people.