(im finishing A Storm of Swords)
My pre-impression was that they couldn't achieve in now way the book level of detail and story.
My impression was good and bad. I loved the photography, and everything, But as I feared, they won't be able to develop the characters story as in the book, nor understand and follow up the houses, the plotting, the backstabs.
As everyone said; Drogo is portrayed in the books as brute, but gentle. The Dothraki are savages, but Drogo is gentle to Daenerys, and that is developed through the whole first book. Also, Robb seemed too old IMHO, Jon, Bran, Sansa and Arya are exactly as I imagined them from reading the books.
The intro is Superb!