This book of NPCs is a great resource for anybody running a Planescape campaign.
Most books of NPCs make the mistake of making the NPCs unique and interesting, but hard to use. Every NPC requiring the DM to run an adventure around the NPC to be able to use them.
Uncaged avoids this by having most of the NPCs in the book act as couriers, money lenders, advertising cryers and a wide variety of shop keepers and other service providers. This allows the DM to use these NPCs without having to center adventures around them. Don't get me wrong there are a large variety of adventure hooks and ideas in this book, but the DM has other options rather then letting the supplement dictate his campaign. The book also does a good job of making the NPCs interesting to read, some are standard NPC descriptions, some are interviews, diary entries, wanted notices, etc. The back of the book also includes a section showing how the NPCs tie together and diagrams going over the major adventure hooks in the book.
This book is pretty much perfect, but I am docking a point, because this book is only useful to people running planecape campaigns. Its a rare campaign setting that has demons, angels and monsters running shops, but when you add in the various factions and Sigil's immunity to gods which most of the NPCs touch on, these NPCs can't really be converted to non-planescape settings.