It's a decent book, though it would've been nice to have a more precise idea of what it is. It provides a wide variety of interesting ideas for storyteller, and a few for players (variations/explanation of character creation, options for replacing virtue/vice, mortal "power stats", a new template: Extraordinary Mortals for making your own Sherlock Holmes/Hannibal Lecter, Social combat, rapport and relationships, a multitude of combat system changes some from OWoD and some brand new, new/altered morality systems, and my personal favorite: several post-apocalyptic systems: everything from mass spirit uprisings to nuclear winter to alien invasion). All in all Mirrors is something of a "Pandora's Box" of system hacks, game mechanics, and things that, generally, only the most twisted/bored/desperate ST would have a need for.
In a sense, Mirrors provides a look at WoD game mechanics that goes far deeper than most gamers are inclined to. With a little effort, it would be a fairly simple affair even to make your own merits, supernatural powers, and even completely new supernatural creatures to turn loose on/as players.
All in all, a good book for very serious gamers who want to add or tweak the basic game mechanics, or make house rules without making them game breakers. But it's about as far from "necessary" as can be since, in essence it is a "roadmap" of sorts to the skeleton system common to all of the NWoD games.