I'm of two minds for this book.
FIRST, the book is good. If I had never read the UA, I would be thrilled with this book, because the fantasy it introduces is pretty intriuging. I'm working on making a setting based on ONLY the concepts found inside of Tasha's for characters (so all clerics are Order/Peace/Twilight, or all fighters become Psi Knights or Rune Knights). So far this has proven to be an amazing exercise and a vivid world, which is half of what I want from any D&D book - the material needed to create cool, vivid, novel, and fun to play in worlds.
BUT, this book drops the ball in some areas. The DM advice chapter is particularly thin. The Session Zero information is good, but basic. The parleying with monsters material sends you literally to the DMG to read it there, which is really lame. The Supernatural Regions are the best thing in this book (and the Magical Phenomena too). The good outweighs the bad here, but then I look at the subclasses, and unfortunate is it that I HAVE seen the UA first. Many of these subclasses have very uninspiring higher level features. The slew of nerfs throughout them is unneeded, I feel. And as for the "variant class features?" Most of these aren't variants, just straight upgrades to almost every class. Then we hit the very paltry selection of new spells and a big stack of magic items which are almost entirely for wizards only.
I've only read through the book once, so I can't give it a "true grade" just yet. However, right now, the book is somewhere between a 6/10 and a 7.5/10 I feel. Above average, good, but not quite great or what I would consider amazing.