I'm sad to say that this book has had no practical use or purpose in my campaigns. It wound up being a gift, so I can't complain of having spent money on it, but I'm still unable to get much purpose out of it.
I don't know if it's a personal feeling, or if it's part of the book itself, but I don't see the "historic" Demons and Devils presented as being either interesting or useful in a D&D context. They just don't fit properly.
The archaic use of 2nd edition fiendish terms (Tanar'ri and Baatzu) is grating and distracting, and the sections about interactions with Demons and Devils in the back also seem to be artifacts of 2nd edition. Things aren't expressed in straightforward levels, spells and costs, but instead long lists of specific components and rituals that makes for good descriptive terminology, but bad mechanics. The random tables are equally frustrating and hard to use. All in all, most of the "supporting" mechanics come across as a distraction, and leave you feeling it's better off to come up with something yourself.
As with many FFG products, there's also a feel of over-specialization of the items they're offering. There's plenty of background, but unless you want to center a storyline around one of these Demons or Devils (as the vast majority are unique), there's not a lot of utility. As with other reviewers, the quality of the stats for the various demons and devils presented varies widely. I get the feeling that this book wants to be occupying "center stage" in your campaign, and if you don't want to put it there, then it's not going to be useful as a sideline.
I have considered the possability of using some of this material in an alternate setting. The "real-world" nature of the fiends presented within might make it suitable for a Call of Cthulu or D20 Modern campaign, and this potential still manages to earn it a 2, but all in all, I think a gamer's funds are better spent elsewhere. I don't tend to think as much in terms of cost to page or cost to content ratios as much as cost to utility...and this book has had zero use.