There are a couple of things to keep in mind for readers. First, this is a history book, written like a history book, intended for a more 'scholarly' audience, at least from the point of view of the fictitious writer Archmaester Gyldaen. Secondly, the book is focused entirely on House Targaryen, who from the point of view of the writer and his audience, are exotic immigrants with unique customs and features relative to the local peoples. Third, this book is only the first half of a two-volume set, covering roughly the origin of the house as Kings of Westeros, until roughly the halfway point of their dynasty.
That in mind, this is a FANTASTIC book. True fans of A Song of Ice and Fire will love this book and appreciate the time and patience Martin had while writing it. The book is a treasure trove of factoids, sprinkled with a few easter eggs of import that effect his other works. If you are a true fan and a constant consumer of all things involving A Song of Ice and Fire, you really MUST read this book. You will love it, if you give it a chance.
On the other hand, this book is not for everyone. If you dislike ponderous books relating the deeds of kings and what they did, you will have a rough time reading through this. Though much saucier and more pleasant to read than actual such works, Fire and Blood is still a lot to get through and people used to the chapter-by-chapter changes in perspective and place may grow frustrated with the much narrower scope and cast of characters. I'd do my best to convince them otherwise, but some people will just avoid this book on principle, and that is a shame.
Looking forward to Winds of Winter, and I hope everyone enjoys this engrossing book!