First let me state that I love this book. I read it all the time, though I haven't come close to finishing it due to its size.
I love this book because it is interesting. It has every British ruler you can think of piled in there, from the most recent to the semi-mythological ones of bygone days. And this includes the ruling families of those who came to power in Britain. The most obscure rulers are covered.
The book is incredibly well indexed. I can find any ruler easier than it would be to look his name up alphabetically. Even better than that, I don't have to know the name of a ruler to find him. There are charts, graphs, geneological tables, maps, everything.
However, there is one major flaw with this book. As the "Royal Book of Records" in the beginning might suggest, the book might more be considered gossip than fact. The RBoR is fantastic, listining the Top 10 rulers for all sorts of things...strangest deaths, earliest marriages, most kids, you name it.
But the whole book kind of takes on this theme.
I would not have known that the book isn't the most reliable of sources if I hadn't gone and tried to use it on my papers for university (Dalriada, Pictland, Vikings, Anglo-Saxons mostly).
Which I did, and subsequently got marked off on. Every point that I cited with this book got marked off.
But I still can't help loving it.