So I wrote a nice long review and between Goodreads, facebook and this iPad that review got lost. Suffice to say, some bits were really good and some bits were really annoying. Overall though, I liked it a lot.
The chapters of Arya and Sansa Stark both annoyed me a lot. They are two girls pretending to be other people, yet the chapters are written as if they've had total personality transplants. Both characters seem far too willing, and far too able, to repress their existing personalities and take on these new ones. At the drop of a hat. With no apparent conflict or any occasional mistakes...
The absence of some key characters is also annoying, and while Martin explains this it feels like an unsatisfactory approach to break what he wanted to write into two books. It didn't work with the publisher splitting book three down the middle either, so I'm not sure what the right approach should be.
I expect that book five will complete book four, in the same way that the second half of book 3 did to the first half.