Pretty much what everyone says about this book is true...
The HBO series is very close to the actual book. Actually almost too close. I was having deja-vu moments as I read, until I got post the episodes I had seen. From that point, I actually enjoyed the book more. Not that either were bad, it was just easier to stay in the moment, in the context of the book and to not try to marry up what I was reading with what I had seen.
What I really liked about the book was that the expected just didn't happen. People you liked died. People you don't, didn't. Stories did not progress they way you thought they would at all. The plot really kept you guessing and wanting to know what would happen next.
This is NOT Tolkien fantasy. While the plot alludes to magic and magical creatures, etc, in this book there was very very little of that. This book was about people, their struggles, internally and externally, battles, death, life and honor.