A Clash of Kings is a great sequel to A Game of Thrones, but I cannot give it 5 stars like its predecessor however. It's very well written and the shifting perspectives and decent chronology aspects never get to bore you as a reader. Nevertheless, halfway the book, I found that most stories and subplots had come to a standstill when literally all kings contending for the realm decided that waiting for the others to attack each other first would be the best strategy. Shortly after Martin took me back in his typical roller coaster and all was well again.
Another mentionable point is that the chapters always end on a very well chosen moment. There are cliffhangers, moral points to decide on and nicely rounded up subplots. This keeps the 1000+ pages a treat to read.
I already started in the third book (A Storm of Swords) and really look forward to the remainder of the story!