If I could give this 0 stars, I would. The product is cheaply made, and countless people have had the pages all fall out of the binding upon opening it as well as smudged ink on the cover and on pages, and even pages bound upside down. The contents is 90% recycled. From art to stat blocks, it's almost all the same as found in previous sources. It also removes most of the lore that describes the history of the monsters, how they behave, etc. It also changes a lot of player races in undesired ways.
There is a small handful of new art, and spellcasters were reworked to make them "simpler" to run but really it's just making the system complicated in a different way. The player races have had a lot of their defining traits completely removed or replaced, and all automatically start with at least 30 foot speed regardless of size. Any trait that was a drawback has been removed and any powerful traits have been nerfed into the ground. WotC is trying to be overly PC and homogenize the races so as to have as little difference between them as possible.
The positives? Most features are now based on PB per Long rest, which grows as your character levels as opposed to tying them to a specific stat you may not want to focus on, but its hastily thrown together and often should serve only as a basis for your own system as it harms rather than helps in some cases. That's it. That's the only benefit. One might argue how it divides actions, bonus actions and reactions is an improvement since Spellcasting falls under actions, but the new Spellcasting system immensely limits how you can use the creature so it shoots itself in the foot there.
If you're a veteran player, don't even consider this shameless cash grab. Its initial release was shameless enough in and of itself, but even just the standalone book is a crime.
If you're a new DM, try and get hold of Volo's Guide to Monsters and Mordenkainen's Tome of Foes. The difference is night and day. If those books have been discontinued and are unavailable at the time you're reading this, there will undoubtedly always be resources online that provide you with the original content.