While not bad, The Year of the Flood was not up to the level of _Oryx and Crake_, and was at its worst where it intersected with the original book. Atwood should stick to her policy of avoiding sequels.
First, the good. Atwood's modeling of psyches and writing about relationships is enjoyable and deft. Even the somewhat hum-drum action that fills most of the book is made quietly enjoyable by this skill. Second, God's Gardeners (GG), the central religion of the book, is a delightfully imaginative and well fleshed out religion. This is one of the best versions of eco-Christianity I have seen, and is the primary reason I gave the book 4 stars rather than 2 or 3. Some of the hymns and sermons are just as good as the originals, and many have a quiet and delightful humor to them. The mix of absurd theology, cheerful euphemism, minor church politics, and unshakeable optimism in these sections was always enjoyable. Adam One was a particular favorite, and his canny intelligence, good humor, and wisdom and seemed like a bright mirror image of Crake from the first book. The Naruto to his Sasuke if you will.
The bad parts of the book are mostly where it ties into the original. In the first part of the book, you have well fleshed out, sympathetic, and very human protagonists growing up amongst the GG's. Unfortunately, they are then dropped into the compound world of Oryx and Crake, and Ren becomes obsessed with Jimmy. Fucking Jimmy. We've had enough of Jimmy, he had a whole book, and even there his best parts were when he was a mirror for the characters and events that surrounded him. We don't need more time spent on Jimmy, and certainly not by characters who are interesting and that we actually like. Even though this particular section ends fairly quickly, all of the parts that deal with the corporate world seemed like something of a second rate re-hash. This aspect of the world was already covered in great depth in the first book. The deafeningly tin-eared names for corporations, products, and tribes are still out in force. Editors, please don't let Atwood do this to herself again. Also, the plotting of the action, especially after the Flood, seemed rather weak. Zeb, the ultra-competent survivalist, is just fine sending out his two malnourished and untested friends on their own to hunt the Reaver-esque painballers. About ~80% of the named characters survive the supposedly species ending plague, and they all find each other in about a month. Etc. etc. Just as bad is what their survival does to all the themes and ideas in the first book. Crake's new creation and Crake's plague were supposed to be two sides of the same coin, with the plague eliminating the human stain that would otherwise destroy his perfect new creatures. If however plenty of humans survived the plague, then that elegance is right out the window, and the entire first book was basically for nothing. I try to make it OK by telling myself that the characters from The Flood will all die of skin cancer in a few years, but that's a wan comfort.
In conclusion, the Year of the Flood is a little flabby, a little overwritten, and has some major missteps, but is at least partially redeemed by a delightful religion and its adherents.