I read the books very recently, because I wanted to have read them before I started to watch the series. Loved the books!
The HBO series is very good IMO. Casting is mostly good, with some exceptions (Jon Snow is not at all as I imagined). The only real problem IMO is that the series does not spend enough time on introducing the characters. There are so many interesting persons and getting to know them adequatly from the series is hard. My wife, who has not read the books, seems to have trouble following who is who and what motives they have etc, and I don't blame her.