" It also didn't make very much money"
Followed all the low budget rules, except one: they forgot to make a horror film.
"the plot "twist" becomes quite obvious early on and the movie moves pretty slow"
That is very true, and the movie also made the decision not to reveal the plot twist in trailers and blurbs, which is rare, as the "plot twist" could fairly be said to occur at the end of Act 1: the set-up. Still, I don't think revealing the whole premise in trailers would have helped, as audiences would have realized anyway that this is a cerebral think-movie, not a balls out thriller.
Personally, I love this movie. It takes echoes of other movies I love, like "2001" (Kevin Spacey is this movie's "Hal") and "Silent Running" (Sam Rockwell is this movie's lonely Bruce Dern), combines them with another standard sci-fi premise, and reinvents itself as a thoroughly effective think piece. The key to the success of the movie is the bond it builds between audiences and characters, before revealing critical information about those characters.
The fact that the director, Duncan Jones, who also came up with the story, was David Bowie's son resonates. There are elements in this movie, such as the degree to which the Kevin Spacey AI successfully replicates human traits, which must resonate strongly with him, himself forever wondering whether he replicates the traits of one of the most original and iconic men who ever lived. What is the essence of humanity and individuality?
Sam Rockwell is low key terrific in this, just as Dern was in "Silent Running." It is his unflashy modesty that universalizes the story, a story not of one exceptional individual, but of all of us. :)