I wouldn't call it a thriller, and certainly not a horror movie. There are a few tense moments, but most of the movie is actually very mellow and slow.
I enjoyed it for what it was, and the few tie-ins with Alien were nice for people that had seen it. No requirement of having to have seen it, like a lot of people seemed to think, and if anything, it brought up more questions about Alien than it answered.
Like I said though, for what I was expecting out of it, I was happy enough and entertained
