Firstly: yes, this film is a massive letdown. It is unquestionably visually stunning (especially in IMAX 3D), the nods to 2001 are very welcome and Michael Fassbender is surely one of the best Brit actors of his generation.
But the writing is beyond awful on pretty much every level. Dialogue is almost universally hammy and often poorly delivered. The actual sequence of events (AKA plot) is frequently nonsensical.
But worst of all (and this very much betrays the LOST background), questions are left unanswered, not in a tantalising and fascinating Alien-ish way (e.g. the Space Jockey itself, which was infinitely more interesting in the one scene we see it in Alien to the many times we see it here) but in a frustrating and unjustified way, either through a misguided attempt to re-evoke the Alien mystique, or just through sheer Lindelof-ish laziness/lack of intellectual rigour. Remember all those analyses of the murals in the ampule room? Our in-depth theories about what they might be? Well I'm none the wiser at the end of the film, not one bit. Why is there effectively a Xeno on the wall of that room? Still got no idea, and the film doesn't begin to answer it.
See, when we were all trying to piece together the chronology, I think in our worst nightmares none of us imagined the answer would be
Spoiler
actually everyone just gets screwed up in different ways with no real coherence beyond "that black sh!t is really, really bad". And still less did we imagine that different segments of the plot would involve characters with no real interest or even apparent awareness of any other. While I'm at it: wtf was the point of David trying to impregnate Shaw via Holloway, then not seeming to care at all that she'd had it removed, still less check what had happened to it? But in any event, what did that have to do with waking up the Space Jockey? And why did they all just let Shaw run around doing what she wanted when she clearly wasn't going to want them to complete their plan?
Also - the much touted secretive ending? I'm not trying to blow my own trumpet here, but I pretty much exactly predicted what it would be (from Lindelof's comments about an "encore") and it was done in the most obvious and uninteresting way possible. Not only that, it doesn't make sense AND it was the one part of the movie that was pretty poorly designed.
What I really can't tally together are the many interviews and promos Scott and Lindelof gave together and the actual film. Remember Lindelof talking about how the "characters in the film are having conversations about Gods" - nope. Well, about one brief mention of Darwin, but there's no coherence or depth, and given the Space Jockey part of the plot seems to just come down to
Spoiler
"they're building a military installation" and that's it, there doesn't even seem to have been a serious attempt to tackle that stuff.
Finally: was this even well directed? Scott is hit and miss for me anyway; this seems to me to be visually up to his best, but the lack of any real tension, and the good actors who go to waste here, have to be laid at his doorstep, surely. The opening scenes with David and the basketball...nicely done I guess. That was about the only scene, literally, where I felt he was rising above being a highly competent director-for-hire.
Sorry to be so negative, but I'm amazed anyone thinks this is even a decent movie. It isn't - it's poor. 4/10.