If you're really interested in a critique, I advise you to read my reviews, neither of which are very spoilerific, of the first two books. All my criticism (and praise) of those books holds for Hybrids as well. It saves me typing and saves you bandwidth and valuable time you could otherwise use for, say, reading books.
Oh, and there are spoilers now. You started reading this review regardless of the automatic warning, though, so I assume you're OK with that.
One of the main plots in Hybrids centres around Ponter and Mary's budding relationship. They need to work out living arrangements, considering that Ponter spends twenty-five days of the month living with his man-mate, Adikor, and if bonded to Mary would only expect to see her four days a month. Mary has to get over her conditioned discomfort with Ponter and Adikor's intimate relationship, and she has to decide where she wants to live—her Earth, or his. Finally, Mary and Ponter want to have a child, and they need to decide if it will have a predisposition toward religious belief (like humans) or be an atheist (like Neanderthals).
This plot is the most interesting part of the book. Mary's ultimate decision to make their child a born atheist is no doubt controversial. I'm an atheist, and even I at first expressed some indignation—I thought Mary's decision was one that she couldn't make, that the child should have the choice. But belief isn't a choice, is it? I can't just choose to suddenly change my mind and believe in God . . . such convictions are deeper than conscious thought. So my initial position seems to be wrong; the idea that Mary and Ponter's child should be born with the potential for choosing religion or atheism because it's an "obstacle" to be overcome is just as bad as saying that the child should be born blind so it can "overcome" the obstacles associated with blindness. Religious people would no doubt disagree … but such a debate is outside the scope of this review. It's enough that Hybrids sparks the debate; science fiction should do that.
The other plot is Jock Krieger's genocidal attempt to infect all Neanderthals with an altered, selective strain of Ebola so as to wipe clean their pristine version of Earth and leave it ripe for human colonization. This plot isn't nearly as convincing nor as interesting as the other one. Firstly, Jock is just such a stereotypical villain—the "avaricious American"—that I cringed a great deal while reading his scenes. Secondly, Sawyer can't maintain the suspense required for the amount of travel his characters have to accomplish just to foil the bad guy.
With regards to Jock, I had a hard time believing someone could be both that nefarious and that blasé at the same time. (I'm sure some people in real life are, but fiction, unlike real life, has to make sense.) His character didn't sit well with me in Humans either; he seemed to fluctuate between earnest scientist who desired synergy and coldblooded game theorist who only wanted to exploited the Neanderthals. And as soon as Mary gives him the Neanderthal codon writer, the first thing he does is manufacture a virus that kills Neanderthals—and only Neanderthals. It wasn't a big deal to him though.
After reading Flashforward and these books, I've realized that Sawyer has a penchant for forcing his characters to traverse nearly impossible distances in very short lengths of time. In the case of Hybrids, he ups the ante: our characters have to go back and forth as their goals change toward the very end of the book, and I had a hard time keeping it all straight.
What most let me down about Hybrids, however, was the fizzle of the threat of Earth's geomagnetic reversal. In Humans, we learned that it was possible that the collapse of the Earth's magnetic field would cause human consciousness to "crash." Not only does Sawyer dismiss this threat in Hybrids, but he does it in an incredibly banal way, tacking it on after the climax where Mary and Ponter confront Jock. All of humanity goes on a great big magnetically-induced acid trip with themes ranging from religion to alien abduction? While this could be an important plot point in its own right, the way Sawyer included it at the end of the book turns it into an afterthought and undermines the intriguing ideas he advanced in Humans about the link between consciousness and Earth's magnetic field.
So what's new with Hybrids? Not much. Babies, genocide, a little uncomfortable dramatic irony. As far as concluding volumes go, Hybrids wraps up the plot nice and neatly, but it doesn't earn any points in the drama department. The story here is thin and not very satisfying.