Lotsa good new demons and feats...
It goes into a lot of depth on a few of the major levels of the Abyss.
Also good treatment of the ways and physiology of the demons.
Finally it introduces a new species grouping of demon.
Written by Erik Mona and James Jacobs, how can you lose?
I'm only sad that there never was a Fiendish Codex III dealing with the yugoloths.

While not 100% perfect (the spells, specifically, didn't really grab me), this book is an excellent resource for anyone that has any interest in running demons in their campaigns.

Facinating information not just on demons and demon lords, but also on the Abyss itself. Plenty of great background information that will be useful to thrown into nearly any campaign, and some cool new monsters as well (not to mention some revision to the CR and information about a few existing monsters).

On top of all of this, lots of 3.5 ties to previous edition material, fleshing it out or at least bringing it to new eyes that may not have seen in in the current edition.

And hey . . . 3.5 stats for a Molydeus . . . how can you not love that?

Fantabulous. Despite the long-standing history of using the baatezu in my campaign, I may have to give the characters a switch-up and study this book most thoroughly to give them a change-up from what they're used to encountering. The oldies and baddies are in here - Baphomet, Grazzt, Juiblex - but there are also ones that I haven't played around with yet - Malcanthet, Pazuzu, Pale Night.

The signatures are a nice bonus as well - I get the opportunity to test my dice theory as well. (Will my dice rolls be better by rubbing my dice on their signatures? :D ) This book makes me crave the Fiendish Codex II all the more.

As a DM, I look forward to doing precisely what Erik Mona's signature said - "Stay Evil."

This book expands on everything that the sourcebooks in the past have said about demons, and goes into marvellous depths. The histories and interactions of the demon lords and the layers get a new coat, and things get explained, or appropriately hinted at to great effect. New approaches to how to play demonic encounters and themes in a campaign abound in the book, several new types of demons, familiar faces getting new coats of paint, several new ones as well.

Some independent reviews nitpicked on stat block inconstancies and errors, but i found everything viewed as an error to be something i could work with or work around.

If you as a DM plan on using demons in your campaign, you would be well advised to jump at this one.