Emily wrote up an excellent summary of the plot of Oryx and Crake in her review, here. I am not going to reiterate that, just give my impressions of the book.
It is a very powerful, well written story. The general theme seems to be a dire warning about where genetic modification and all that fun stuff might lead. As Emily mentions in her review, the story takes place in two settings... the time before a mysterious virus kills most of humanity and the period after. There is a single POV character in both periods, reminiscing about various events in his prior life, and sort of wandering around the setting complaining after the virus.
I was much more interested in the parts of the story focused on the time before. Until the last few chapters, I was waiting to find out just what had wiped out humanity, and where the POV character's friends Oryx and Crake were. It gave a lot of hints, but Atwood doesn't tell you exactly what happens until the very end of the book. I found myself significantly less interested in the parts of the story from after the disaster. The main character was just wandering around and complaining, I felt like. It didn't add a whole lot, just made me want to skip ahead to the parts about the world crashing and burning.
This book is described as about a love triangle. That is totally false advertising. There may in fact be a love triangle in the book, but it only exists in the last few chapters of the book and seems fairly contrived. Just to sort of maybe almost explain a certain event. Not really central to the story at all.
My main complaint about Oryx and Crake is that I was only ever emotionally attached to one character (Killer) who was both minor and killed off early. I found Jimmy/Snowman obnoxious, Oryx unlikely and contrived, and Crake just immensely unlikeable in general. Although not to the extent that I thought his ultimate contribution to the situation in the story seemed very plausible.
I am not sure how comfortable I am with Atwood's general anti-bioengineering message. I think there is valid progress that could be brought about through bioengineering! The chicken nobs, for example, seem like a good innovation! As a vegetarian, would I eat chicken that was grown on what were essentially plants that felt no pain and had no brains capable of suffering? Hell yes I would! I think that I would sign on to human organs being grown in pigs if it could save the life of someone I loved. I think a lot of valid progress is to be had through these avenues, and I took issue with the way Atwood treated these ideas as self-evidently bad.
The setting of Oryx and Crake seems to be devoid of both morality and a real government. It seemed like kind of corporate fascism/anarchy where money was power and freedom only existed for those with money. Obviously, in this fictional setting devoid of any ethics or oversight, corporations ran wild with bioengineering. But that seems realistically more of an argument for a strong, independent FDA than against the science of bioengineering.
So in all, this book is interesting and raises important questions, but is not without a significant bias, in my opinion. If you are already against bioengineering or whatever, you will probably appreciate it more than I did. I don't think the story was compelling enough to stand on its own as literature without the social commentary, and the social commentary was a little much for me. Not a bad book, and I enjoyed reading it, but it isn't something I would necessarily recommend to most people. Now I want to read Margaret Atwood's The Handmaid's Tale again and see if I am as crazy about it now as I was in high school!