I am playing through this at the moment, and the Official reviews (GameSpy, PC Gamer) got it bang on. Lacks all the atmosphere from AvP2. I am interested in the back story, but I don't think I'll follow this through.
If you found the first game to be very challenging, this expansion has the potential to frustrate you to no end. It also lacks the elements that made the first game suspenseful and packed with atmosphere.