I have read all the books and seen the 2 seasons. I really love that series.
I thought season 1 was brilliant, they knew how to adapt the book by making a few minor changes, while at the same time being really true to the books.
Now, I was a little disappointed with some of their choices for season 2. It was amazing too, but I thought they were going too far with the omnipresence of sex. Seriously I don't mind some sex and violence, and there is already a lot of it in the book, but I think that precisely, there's quite enough, no need to try and add even more whenever there's an opportunity... They were also making some really unnecessary changes, and I even thought that some of the dialogues were poorly written (Danaerys and her great speeches, or Theon Greyjoy who had some really bad lines)
In the adaptation, what I dislike the most is their tendency to try and make absolutely EVERYTHING clear and explicit (especially if it involves more sex).
What I mean by that is, the book has a lot of small mysteries (often regarding who slept with who etc) and Martin is careful not to say clearly whether one hypothesis is truer than the other.
Examples include: -Renly being gay and his implied relationship with Loras Tyrell (that's what I bothered me the most, mainly because I liked Renly and they completely changed his character to turn him into a blatant gay stereotype)
-Stannis and Melisandre being lovers
-Margaery Tyrell's claim that Renly did not consumate their marriage
-Also Margaery's attitude: in the books, you never really know whether she is truly a very chaste girl or if she's just a libertine who sleeps around with everyone and is just good at hiding it. Now I have a hunch that in the series, they're definitely going for the "slutty" option...
Otherwise, I love everything else. The soundtrack, the settings, the costumes are absolutely perfect. And the cast is really very close to perfection, as well (I agree with other people that Arya in particular is incredible).
What I love the most about A Song of Ice and Fire is the quality of the characterization. None of the characters is 100% lovable, and yet, you just love all of them. I mean, it would be impossible for me to select one favourite character, and when I start thinking about it, I realize that I love ALL the characters.
Ned, Catelyn, Cersei, Jaime, Tyrion, Littlefinger, Danaerys, Arya, Sansa, Jon, Stannis... I don't know, in their own way, they're all perfect. Even the bad ones are perfect, I can't say I "love" Joffrey, but God, do I love to hate him!