This is one of the best RPG releases of 2011-2020. Easily.
They somehow managed to do with this book, what many RPG publishers simply forgot how to do:
It offers great fluff. I was someone who was antagonistic toward AoS at first, because I felt that GW killed of the flavourful Old World for bland new nothing just to sell Fantasy Space Marines. And while this might have been true a few year ago, I feel that AoS ALREADY has more potential for interesting stories of a much wider range than the Old World ever gave us.
The setting info in this book is not only found in the setting chapter (which features the best overview of the Age of Sigmar timeline/setting that's available anywhere right now, while also giving story hooks and adventure seeds in almost every paragraph), but also inside the introduction and the archetype overview. This is done without ever mixing up fluff and crunch in any way, and it's also never confusing. The book flows from "broad setting overview" to "that's who you will be playing" into "and that's the world they live in".
It gives the players a wide range of options to create characters that are /just/ right, and serve both the "let me just play" and "I want to do everything myself" players. You can start the game by choosing an archetype and begin playing after a few choices, you can chose to customize an archetype and take longer, or you can chose to go fully free-form character creation.
And it offers solid, well designed crunch.
Character skills and talents cover a wide area of expertise that helps build characters that feel different from each other without overwhelming players with lists upon lists of rules exceptions and tiny additives.
The combat system manages to hit both "meaningfully complex and rewarding tactical play" and "crunchy but not overwhelming" states.
There's a Magic system with a wide range of spells that is easy to grasp and use, and also a really great system to create new spells, that is intended be used by PC spell-casters and not just the GM, giving players a hand in further developing their characters and the world they live in.
And then we come to the "endeavor" system (downtime/between adventures actions). This was a great and positive surprise to me. It gives a group of PCs an intrinsic connection to the area they operate from, and allows them to shape its future and strengthen its chances of survival. It's also another system that's easily customizable by the group and the GM.
Coupled with the Doom Pool mechanic, the game uses all those options and capabilities to really tie the characters to the game, taking care to give players a hand in shaping, defending and bleeding for the people who see them as heroes.
The book also offers a really well written and presented GM advice section in which it talks how to change the tone and scope of the game, which was another welcome and surprising addition. All those preferring a more Rat Catcher like tone and feel should sneak a look into here. But those who want to enter into higher levels of play faster will also not be disappointed.
The Bestiary not only gives stat-blocks, but also ideas and suggestions for customizing the adversaries and how to use them in sessions/campaigns.
So, not only is this a really damn great RPG, it's also one that's more than just complete.
While I'm looking forward to sourcebooks deepening the info on some aspects (Lumineth, Seraphon, Vampires and Orruks as the four main ones, but great Skaven content would also be quite excellent.), I could easily run several years worth of campaigns just with this book. There simply isn't the feeling that something's missing.
Both thumbs high, HIGH up for this one. :)