I don't know if Fire & Blood was good, but without a doubt, it was impressive.
George RR Martin's Fire & Blood is a hard book to categorize and therefore it's also a hard book to review. It's fictional, but it certainly isn't fiction in the typical sense. Instead, it's a made up history book written by a made up historian, covering the years beginning with Aegon's conquest of Westeros up until roughly 140 AC (that's "after conquest" in case you aren't a deep Martin aficionado). For reference, the events of Martin's A Song of Ice and Fire (and the HBO show Game of Thrones) take place roughly 300 years after Aegon's conquest, so I'd imagine there's one more book coming to finish up this Targaryen history (perhaps it's time to give up on Winds of Winter?).
For fans of fantasy literature, the book that probably immediately comes to mind when thinking of a fictional fantasy history would be Tolkien's The Silmarillion--and there are obvious parallels. However, whereas Tolkien's prequel reads almost like mythology or a religious text (it begins: "There was Eru, the One, who in Arda is call Iluvatar; and he made first the Ainur, the Holy Ones, that were the offspring of his thought..."), Martin's history reads like... well... a history textbook. If that sounds incredibly boring, I wouldn't blame you; at times it is. That said, while the prose might be a bit dry and at times Fire & Blood can devolve into a litany of names of knights and lords and battles and places, it is a masterful piece of world building that is pretty much unmatched in literature (including Tolkien).
This may sound a bit sacrilege to some fantasy fans in that Tolkien is pretty much held up as the Michael Jordan (aka the GOAT) of fantasy world building--and for good reason. Prior to writing LOTR, Tolkien had mapped out literally thousands of years of history in Middle Earth. The difference between Tolkien's and Martin's world building though is that most of Tolkien's historic characters were black and white/ good and evil, and so creating a history is simply an act of recounting what happened. Martin's history in Fire & Blood, though spanning a far shorter period of time, deals with characters who are almost entirely gray and human. And so, in recounting what happened, he also needs account for all of his historical characters on a very human level: what were they like, what were their motivations, etc. and how did each character's motivations interweave in an endlessly complex series of cause and effect reactions to create the world that eventually became the one we saw in ASOIAF. By the end of the book, I was floored by how Martin was able to pull all this off.
Bottom Line: Fire & Blood can occasionally be tedious, but it shows a master world builder at the peak of his powers and makes me excited for the potential of the forthcoming HBO show ostensibly based on some of this same source material.