Given that it was essentially supposed to be a history book, I thought it was quite good. A few interesting bits of information that might affect the main storyline (though I doubt it) and as a whole Westeros, especially the Targaryen reign is fleshed out nicely. Asshai and the other far eastern parts of Essos appear to be quite creepy. It's not a tell all book though, if you're expecting that. It's written in the same way his previous two books are written (Princess and the Queen and the Rogue Prince), from the PoV of a Maester. It's basically a wiki with nice pictures.
Of course given that the story is still incomplete, not everything is revealed. There's next to nothing new on either Robert's Rebellion or Summerhall which he's probably saving for the main book and the D&E respectively.