While Creature Codex is very similar to Tome of Beasts and even superior in a few ways, the market of 5th Edition is very different now than when Tome of Beasts had released. Since mid-2016 we have seen two monster expansions from Wizards of the Coast: there’s simply a lot more monsters in the game. There isn’t as much of a burning need for this product. While Tome of Beasts quickly became a must-have book that was used every session or two of my home game, I have still used only a small fraction of the content from that book. I doubt Creature Codex will see as much use. The competition for table time is just higher.
Counting through the monsters of the book, there are 115 or so that I’d use without hesitation, and a few that might be dropped into my game sooner rather than later. And slightly more than that are what I’d consider “okay”: monsters I might use if they fit the story I have in my mind or a fall upon them randomly, but nothing I want to purposely design a story around. That leaves a comparable 110 monsters that are simply “meh”. The monsters I’m unlikely to ever seriously consider. The hit : miss ratio for the book isn’t bad, but it’s worse than Tome of Beasts.
If you’re one of those DMs who thinks you can never have enough monster books, then you should buy Creature Codex right now with zero hesitation. There’s a few surprises, some nice monsters, and the presentation is solid. Also, if you need monsters right away and Tome of Beasts is sold out from your FLGS while a copy of Creature Codex is sitting on the shelf then you will get your money’s worth. It is a good purchase.
But… if you already have Tome of Beasts you’re probably better off getting one of the other official monster books, such as Volo’s Guide to Monsters. If you already own those books, then Creature Codex might be a little superfluous. It’s good, but chances are you haven’t used all the monsters you already own. It’s the monster equivalent of stacking a few new movies onto a Netflix watch list or videogames to your Steam library.
The Creature Codex is a lovely “extra cash” purchase when you have a few extra bucks from a gift or minor windfall. Especially with fewer official D&D books, it’s a nice option and treat for the ol’ Dungeon Master. It’s nice to have and you won’t regret the purchase, but it’s not so necessary that you have to budget for it right away.
TLDR - A great sequel to Tome of Beasts that manages the tricky task of extending and improving on its predecessor's mission to bring more game-ready interesting monsters to 5E.
If you find yourself needing more (or more interesting) monsters than WOTC and/or Kobold Press' Tome of Beasts provided, but hesitate over third-party books; fret no more. Creature Codex improves on the already almost-WOTC level of Tome of Beasts with more monsters, more interesting designs, and the same high-level of polish Kobold Press brings to their top-tier books.
I can personally attest, having run most of the monsters in WOTC's Monster Manual (and a few other WOTC products); many monsters from the Tome of Beasts, and a few dozen from the Creature Codex that this book is game-ready. No futzing around fine-tuning the monster or scratching your head (how is THIS CR7?!?)
The professionally commissioned monsters are as-good or better than their peers in other books. But the Kickstarter patron submissions (46 in total) are what put this book over the top. The sheer variety and ingenuity of monster design in Creature Codex eclipses any other monster book I've seen and makes for a rich mine for DM's to plunder. Examples include Bar Brawl, an ingenious conversion of a bar fight into a form of swarm; Ink Guardian, one of a whole series of mimic-like defenders of literacy; and the Mytholabe which I refuse to spoil. I have a soft-spot for the Mandriano which in (my) original submission was much creepier.