I have to say that the best book in the series is the very first one, Feed. It was fantastic. The second book was still pretty good. I think Blackout is the weakest of the three because of some of the choices that Grant makes. Two of these choices in particular make it hard to call this book a completely satisfactory end to the series. The first is a revelation about Shawn and Georgia's relationship. It came completely out of left field for me and didn't and anything to the story. Rather, it subtracted from the story for me. The second thing that bugged me was how long it took for Shawn and Georgia II to meet up. The real action takes place after they join forces and, unfortunately, this only accounts for about the last third of the book.
I would still recommend this series for zombie fans. It's fresh. It's different. And there's the delightful bonus of snarky dialog.