In a nutshell - I LOVED them. Correction - I loved everything but book 4, but can see the merits of why he did what he did there.
I don't want to post a whole bunch of information about the plot lines because I really think these are worth experiencing without that. If you are eager for spoilers - there are plenty of those online elsewhere.
Of the 5 books, book 4 was the only one that came as a disappointment to me. The characters were all "new" and it was very hard to concentrate on the story. It was as though the author had given the world a spin and this was what had fallen out. It was only when I read the introduction to book 5 that I realized that book 4 had it's own significance to the story as it was occurring during the same timeline as book 5. So, if you have been struggling through book 4 on your own - know that you are not alone (hubby is struggling with it right now too)
As for how closely it relates to the HBO series - the series remains pretty true to the book. They have taken certain liberties here and there - though not as drastically as they did with True Blood/Sookie Stackhouse. At least the series still feels like the story in the books.
The thing I think I love most about George RR Martin's series is that no character is particularly precious to him. He will not hesitate to kill off a seemingly main character if it will benefit the story. There were many times that I gasped at the death of a character - feeling as though the story could not go on - and then became engrossed in another characters storyline and let go of the old one.
One thing that really surprised me about the series is how well Martin crafts strong female characters. This is not something you often see in a series like this. He has several main characters who are bold, strong and fearless girls/women. Considering they were written by a man, I think he deftly crafted these characters into believable heroines.
So, my verdict, in case you couldn't guess, is that I highly recommend this series. You don't even need to be a "fantasy buff" per se to enjoy them. There are fantasy elements strewn throughout (dragons, supernatural beings, animal possessions) but most of it will feel like a medieval war novel. Most of all, though, it is just good reading.
What are you waiting for? You need to go pick them up today and get started!
On a side note - I feel sorry for the book I am reading now...it's hard to leave a super rich world like martins and move into a healthy lifestyle book without being slightly disappointed...