This series contains 3 books: Prey, Hunter's Planet, and War. The first and third are absolutely spectacular. The writing is fantastic--beautifully descriptive, with believable characters, and most importantly, they fit seamlessly with both the Alien and Predator universes. The second is a different story--Hunter's Planet certainly isn't the worst book I've ever read, but it's far inferior to the other two in this series. It is an enormous let-down, both in terms of writing quality and just plain fun. It makes all Machiko's adventures in Prey seem meaningless, and the behavior of both the Aliens and Predators just doesn't feel right, doesn't feel like things they would actually do. (Note that the first and third have the same author; the second is different.)
This is going to sound peculiar, but I recommend reading the first and third and skipping the second, and not just because of the differing quality of the novels. The events of War actually take place between Prey and Hunter's Planet, and War is perfectly readable without reading Hunter's Planet. In fact, several scenes that were told in flashback in Hunter's Planet are RETOLD in War, only told far more descriptively, and even with important events completely altered! If I had the chance to start over, I would skip Hunter's Planet entirely.
I have read many science-fiction novels, and I rank Prey and War among my absolute favorites, worthy members of the Aliens and Predator franchise. On the one hand, it's like getting more of the movies--more terrifying bugs, more awesome Predators, more battles, more excitement--and on the other, there is a lot of fascinating new information about the culture of the Predators, things I always wanted to know but never thought I would. I love the star character, and appreciated the fact that she wasn't the typical mindless sci-fi babe. If you love Aliens and Predator, you might find Hunter's Planet mildly interesting.
(As a side note, the AvP movie borrowed a LOT from these books :)