If you read and enjoyed the Archmaester Gyldayn stories and enjoyed them, then you should be pretty prepared to read this history of Westeros. I say that because while I found the events covered in those stories to be interesting, the narrative itself was pretty dull. Part of that reason was because Martin's characterization in the main series is what makes it shine, and in the Archmaester Gyldayn stories, he removed the characters and gave a summary of what happened. I found myself thinking that I would have preferred to read new stories involving these events instead of the dry summation, and I feel the same about The World of Ice and Fire, too.
My guess is that this book was written mostly by Garcia and Antonsson, with the parts about the kings written by Martin. I say this because the styles are different between one section and the next, sometime obviously so. There doesn't seem to have been much effort made to blend the different authors' styles together, which becomes obvious as you read more and more of the book. Martin's (presumed) sections have a better feel and sound than the other sections, namely because Martin does this for a living, and it should sound better. I'm sure Martin lent his own knowledge to the other sections where necessary, but I wish he had also done some editing to make everything sound like one voice.
The book is ostensibly one written by a Maester some time before the events of the Song of Ice and Fire series, which makes sense, since the current events are covered in those books. What makes it a little odd, though, is the way that the narrator speaks of this ancient history, some of which is prehistorical, and casually dismissing some of the myths from that time, even as we know, as readers, that these things are real. Then there were the sections that were patently false, like in the section on the Vale, the narrator mentions an age of peace due in part to Jon Arryn being the Hand of the King. It was like the author was trying too hard to make the narrative authentic, instead making the tone come across as disingenuous. It wound up being ironic foreshadowing in reverse, and was somewhat annoying.
Overall, I wouldn't recommend this book to any but the staunchest of fans of Martin's series. Most of what is contained in this book has been hinted at or revealed through the story itself (the section regarding Aegon II was a retelling of "The Princess and the Queen, or, the Blacks and the Greens" published last year in Dangerous Women), so most of the content will already be familiar to those who read the books. I mentioned to a friend of mine that I was reading this, and he asked me if it was more a cash grab than anything else, and I have to admit, that's what I take it for. I'm glad to have it as a reference for the series, but I don't think it's a necessary read by any means.