This book fits into the frame of the Vampire supplement Ghouls and the Werewolf supplement Kinfolk. Somehow, in the minds of most, it hasn't become as iconic, and people out in the wild need to constantly be reminded that it exists. I recommend it regularly, and it's a solid book, though largely superseded in ways that the others haven't been.
The sections covering mundane allies and companions for mages has been largely subsumed into Gods and Monsters, as have the sections on Familiars, though it has valuable discussions of how the various factions handle these things differently. The discussion of more aware allies sits in the Sorcerer books, which present many more varieties of Numina.
Honestly, the main problem is that the book is dated. An update to it would be essential reading for running a Mage game.