I wanted to try and do a spoiler-free review for this, but given the nature of the story and the points I want to raise about it, that would basically be impossible.
First, the good — I thought this was easily the best book of the trilogy. Some readers may not like the fact that there are literally no Aliens until around the halfway point, but personally I thought this was a good move. The first half takes its time setting up colony life and introducing us to the major players, and it pleasantly reminded me of the world-building Moore did at the beginning of Alien: Sea of Sorrows, which I thought was the strongest part of that book. Russ, Anne, Timmy and Newt are handled well, and Brackett was a decent enough (if perhaps a little bland) hero. I also really enjoyed Golden's writing style, and again found it to be the best of the three. I hope he gets to do more work in the franchise.
Now for the bad — the book was riddled with little continuity gaffes when compared to the second film. None of these were huge, but they were made all the more annoying by the fact even a casual viewing of the movie would make them obvious. For example, Golden recreates several scenes from Aliens as part of the story. I liked this, it was a nice touch, and for the most part he did it well. But in quite a few of them he totally misquotes dialogue. Surely you'd make sure something as basic as that ties up? Then there's the fact Al Simpson has a moustache in the book. Why, when he so clearly doesn't in the film? Given that the mistakes mostly revolved around scenes and details added in the extended Special Edition, it almost felt as though Golden hadn't actually seem the longer cut of the film, or was at most working from memory. At one point, Simpson also says he's sent a distress message to Gateway explaining the situation. But if that's the case, why does no one, Burke included, know what's happened at Hadley's Hope in Aliens?
Also, I hated how characters quite often made huge leaps in logic as though they'd watched the films beforehand and knew exactly what was going on. For instance, Brackett is told colonists are being abducted by the Aliens, and the moment he hears this he immediately knows they're being taken to a Hive somewhere to be used to breed more Aliens. Yet no one has seen anything to suggest this is what might be happening. So how is he so sure? Likewise, another Marine sees a live Facehugger in a jar, and he immediately knows the Weyland-Yutani scientists have been killing colonists to get them off. Yet he has no evidence to support this. Of course, we know both of these things are true, but then we've seen Aliens. These guys haven't, so how are they assembling these puzzles when they have no pieces to put together?
On the subject of the Weyland-Yutani scientists, they went from potentially fascinating characters in the first half of the book to groan-inducing evil bad guy caricatures in the second. Reese was the worst. Once the poop hits the proverbial fan, he is no safer than anyone else on Acheron. He could literally be taken by the Aliens at any point. But does he care? Nope. He's too busy talking about how they've done wonderful science, being a silly corporate bad guy stereotype. Burke was a corporate bad guy stereotype, but at least he was emotionally affected by the horrors around him. He was scared. Reese acts like he couldn't care less, and it's stupid and unbelievable. Something like that is fun in a video game, but not a novel.
One final note of disappointment — it was great seeing more of Russ, Anne, Simpson and Lydecker, but I felt slightly let down that John Marachuk (the guy who's medical chart Bishop reads from when the live Facehuggers are found in Aliens) wasn't even mentioned. Far more disappointing was that the live colonist the Marines find in the Hive in Aliens didn't feature either. Given that the book confirms Chestburster implantation takes around a day, she had to have been on the run until shortly before Hicks and co. arrive, yet the book suddenly jumps several weeks from when Newt's mother, brother and most of the other colonists are killed, to the Sulaco arriving at the moon. I thought that was a huge missed opportunity.
In summary — I'm aware the negatives section of my review outweighs the positive by about four to one in terms of volume, but overall I enjoyed the book quite a bit. It's definitely worth a read if you're a fan of the franchise.
Lastly — like the other two books, this had an open ending, with characters heading on a continuing journey that will likely be explored in the future. It seems Fox are doing their best to keep their options open.