Okay, so I reviewed the first book here. A positive review. I liked that book, so I went ahead and got the next 2 books of the series to listen to. And, well, the first book is certainly the best out of them. The first half of the second book, Humans, was exciting, and the last half of the third book, Hybrids, was good, but the rest seemed to be rather annoying to me. It became very relationship-y with Ponter and Mary and got quite boring and frustrating to listen to Mary's insecurities and them working out the details of their complicated relationship. Also, I found Mary's religiousness to be frustrating too, she was a Catholic despite being a geneticist and understanding life at a molecular level, she didn't agree with the conservative stances of her church, and even when proof was evidently presented to her, she still denied it and she struggled with the fact that the Neanderthals never had religion, and she couldn't really defend against Ponter's argument that religion had allowed Homo sapiens to justify our wars and violence and awfulness, and instead she stubbornly just said she knew there was a God... As a scientist, I would have expected differently from her character. At the end, she did seem to reconcile her religious beliefs with scientific/worldly beliefs, but it took a world-wide event to make her do so.
Anyway, to sum up my review: If you liked the first book and want to leave yourself with the wonder of another parallel world, with all the wonderful possibilities of a different culture and government, and a society not founded on large-scale agriculture, and an unspoiled earth, you might want to stop after the first book. The second and third books get more complicated, and like I said, relationship-y. If you just want to read about Neanderthal sex, get to like the first half of the second book. Otherwise, it kind of stagnates after that.
Spoiler alert: At the end of the third book, I'm disappointed that there seemed to be no consequences to the whole 'trying to wipe out the Neanderthals from that earth' thing, because it wouldn't have happened at all if Mary hadn't brought the stupid codon-writer into her earth, despite it being banned in the Neanderthal earth! I mean, the High Grey Council (the Neanderthal government) had banned it for a reason! And yet she still got to use it to make a baby, despite the fact that it almost obliterated the whole Neanderthal species, and did end up killing 2 people. Okay, okay, I know that the codon-writer was just a machine, but still, were there no consequences for it getting into the wrong hands?!