Back in the late 70's or early 80's, TSR (for those of you who are new, TSR owned D&D before WoTC) created a book of NPCs called the "Rogues Gallery". It was page after page of lists of stats for characters of each class... Boring.
However, in the back of the book was 10 or so pages of fully fleshed NPC's (mainly from Gary Gygax's campaign)... and that was the GEM hidden in the trash.
Enemies and Allies is the modern version of those last 10 pages... And it is a bit of a Gem itself.
Only 64 pages in length, this booklet packs quite a punch.
Each section is divided by evironment (Crime, Religion, Mages, The Law and Adventurers).
Each chapter provides a cross-section of fully detailed characters for the Niche they have been assigned. There are unique spells and some new Feats that appear (as one might expect), additionally some of the NPCs run beyond interesting into the realm of inspired (such as the Illithid Assassin Yarrick Zan).
The real meat of the book is in the Two-Page "One-Minute NPC" section.
Finally, the "Iconic" characters from the Players Handbook are provided (Redgar, Tordek, etc.) Which is just fun.
Surely this is not a necessary addition to a Game Masters arsenal, but it is a happy addition for those of us that like to see how someone else thinks.