Half way through the first season it might be a good time for a quick overview of how the ratings are doing. Our pre-premiere speculations post can be found here. Below, the ratings of the first six Game of Thrones episodes are shown relative to the first season numbers of three other successful HBO shows.
Note that the ratings curves over seasons longer or shorter than ten episodes have been adjusted to fit the graph accommodating the 10-episode Game of Thrones run. As we have noted several times before, these first airing numbers are neither the only nor the most decisive measure of success, but are still indicative of how the show is trending in general.
Hear Me Roar: Nothing much to add. You will notice that the numbers are consistently higher than the corresponding first season True Blood numbers, catching up with the Boardwalk Empire ones (while The Sopranos aired at a time when HBO had more subscribers than it does today). I would say that if the remainder of the season continues the positive trend and brings about an additional viewership uptick with its exciting goings-on (to use an understatement), the ratings as a whole will be considered an unqualified success. The first part of the answer is coming later today.
UPDATE: According to James Hibberd, the Game of Thrones ratings will be out tomorrow. They are delayed a day this week due to the holiday.