Overall, it was very good. The acting, visuals, plot, and pace, were all excellent. Perhaps the strongest element of the episode is the way it was able to generate sympathy for the main characters, even though almost all of them are actually awful people. This is possible, in part, because many of their most horrific deeds lie in the future. But it's an achievement nonetheless.
The series also turns King Viserys I into a more substantial and compelling figure than he is in the book. This can be defended on the grounds that the book narrator isn't unbiased.
Matt Smith's character may be the only major character who comes off less sympathetic in this episode than in the book.
The younger actress for Rhaenyra did a fantastic job. In the first episode, Rhaenyra is 8 years old and Alicent is 17. They have obviously departed from the book on this point. Rhaenyra is obviously older here and she and Alicent are about the same age. Similarly, Sansa and Arya Stark on GoT were also older than their book counterparts.
Overall, this is a strong start to the series. It's too early to say how good it will be overall. But I look forward to seeing the rest!