As an avid fan of the Alien films(1-3 that is), I thought it was great.
The acting was solid and the characters were likable, it indeed answer some questions from Alien very well. I honestly enjoyed the ending(I won't spoil it, but if you're a fan of the Alien series, then you'll find it rather satisfying).
This was one of the few recent films, where I actually walked out of the theater feeling like it was well worth the show(the other time being The Amazing Spiderman).