This is an impressive and compelling series, very readable and approachable, despite its length.
This is where the series really shines. Martin has created perhaps the largest collection of detailed and three-dimensional characters that I have ever encountered. What is more the characters are believable and their actions make sense to their characters and they grow and develop in believable ways. In the 5th book, however, a number of major characters seem(to me at least) to start 'spinning their wheels' about some thought they have and fail to progress, or grow, or learn from mistakes like they previously had.
Martin has created a fascinating world which seems both foreign to our own and yet very similar and understandable. In particular he incorporates 'Fantasy' elements very seamlessly into the world. The first three books give tantalizing hints and slowly parcels out information to build on. The 4th and 5th he seems to be focusing more effort on world building and has fleshed a good deal more out.
This is where things get tricky. Originally this was supposed to be a trilogy and especially in the first book one can see a lot of buildup for the threat from the North. However, the bulk of books 2-4, and a large chunk of 5, are dedicated to the political intrigues of Westeros and another character trying to learn how to be a good ruler, mixed in with the occasional hint of the greater danger which threatens the entire world. As far as I can personally tell, the problem is that the characters have taken over the story(Martin has indicated something similar) and though the 5th book seems to have the divergent plot threads coming together at last, 5 books in with 2 more to go and I have absolutely no idea what the point of the story is any longer, nor do I have any clue where it is headed. Twists and mystery are one thing, but I am so completely clueless I have some personal doubts that Martin himself truly knows where this story is going.
I do love the series, especially books 1-3, and I sincerely hope I am wrong but at the moment the Jury is still very much out on whether this will end up as a Fantasy Masterpiece, ala Lord of the Rings, or simply a very well written and fascinating character study that failed in its initial promise to be something more.