I think HBO has done a tremendous job with the adaptation. Many of the best scenes in the show were not in the book, such as Varys and Littlefinger staring at the Iron Throne in the final episode of Season 1, or Cersei, believing that the city had fallen, about to poison Tommen in the Battle of Blackwater. All the scenes with Tywin and Arya from season 2 were made up. She never met Tywin Lannister in the books, but the scenes in the TV show were incredible.
The Queen of Thorns is much more important in the TV show than she was in the books.
A Storm of Swords (book 3) is my favorite in the series, followed by A Dance with Dragons (book 5). I think this season has been great.
I have a suspicion about where they will cut off Season 3 relative to the Book. Book 4/5 (it is really 1 book broken into 2 parts) actually consolidates plot lines to some extent. As I have said before, I think HBO broke Storm of Swords into 2 seasons because Martin is so far behind that they want to give him another year to get the 6th and 7th books out. I read somewhere that Martin is even hedging on finishing the series before HBO catches up.
The problem is that Clash of Kings better warranted being split up than Storm of Swords, but it wasn't, with HBO instead just chopping out several battles. A Feast for Crows does not warrant even a single full season by itself. Not enough happens. If they take the last 1/3 of ASOS and add it to AFFC and ADWD they can stretch those out 3 seasons. Which means Martin has to have Winds of Winter done by 2016 and Dream of Spring done by 2017. He has an outside shot at the first deadline, no chance at the second.