Realms of the Crawling Chaos is a fantastic supplement for any OSR game, which not only focuses more closely on Lovecraft's own material v.s. the expanded universe created by later authors, but takes the material with less kitsch and more seriousness than what you would expect for a "add cthulhu to D&D" supplement.
I have not run any of its content yet so unfortunately, I cannot speak for the balance but I can speak for the writing and content, which is pheonominal to a large extent. The book restricts itself to only entities introduced in Lovecraft's own work, his collaborations during life, and his contemporaries whom he referenced during his lifetime, so don't expect Byahkees, but do expect Clark Ashtom Smith's Tsathogguah, or the White Apes from Facts Concerning the Late Arthur Jermyn and His Family.
There is quite a lot of material curbed from the often unexplored earlier works of Lovecraft, and as a result the feel of it is quite different from what you traditionally get from a Lovecraft supplement, focusing far more on the sincere fantasy aspects of Lovecraft's work than the science fiction.
What results is a world that is just as lethal as any OSR game, but with new unknown wonders that offer variety from the typical Tolkein-esq tropes. It also offers a considerable amount of advice for getting the tone and themes right for a game with these concepts. This advice is considerable, and the book is deliberately designed to suggest a setting for keepers rather than give you an entire hard locked setting to play around in.
There are new races, both for basic and advanced LL, one of which is secretly a group of Innsmouth hybrids (for extra fun, don't let your players know what the Sea Bloods ARE except that they are effectively a cleric race class, and then run the generational mutations as time goes on and let the rest of the party slowly realize what they are), another are white aps or white ape hybrids. All of the new races are quite interesting and offer up a lot of potential roleplaying scenarios of prejudice and paranoia, but what is fascinting about this is since the players play as these types they get to experience the horror of this themselves. It's uncomfortable, and not for everyone, and I can definitely see many tables not being comfortable with a player race called "Subhumans", but the roleplaying opportunities for exploring gritty dark and uncomfortable topics are fascinating and adult without ever really drawing attention to it directly. For example, in the alchemy section there is an "Oil of Sea Blood revelation" which could easily be used by an angry mob on a Sea Blood PC hinting at much of the potential prejudice and mob rule that could come about if the secret of their blood was ever revealed. Similarly, Subhumans need to masquerade as ugly humans or face persecution and death. My only gripe about these new races, is white apes seem a tad too exotic to give to players straight out the gate from the beginning, and that the basic versions of these races might not always have enough benefits to make up for the considerable disadvantage they may have socially due to their race. I also wish there was a basic race which functioned as a "magic-user", as although Sea Bloods work as Clerics, humans would be the only ones in basic capable of casting magic. Maybe they are the only ones that foolish.
I decided that when I run my own game of this, I am going to omit elves and halflings from the setting, and make dwarves more of a subterranian semi-xenophobic culture, pale skinned, who's eyes are so adjusted to the darkness that they need tinted goggles in any light more than that of a torch. They are also the natural enemies of ghouls.
The monster bestiary is nice, and the new magic system of alchemy is quite interesting. I am surprised it took any game this long to create a spell for the "spirit bottles" from lovecraft's The Terrible Old Man. My only gripe with monsters is the truly laughable illustration of a mi-go, but it is a part of the art which is doing a great job aping that early TSR style, and at times does a good job going for the style used in 1st Edition Dieties and Demigods' Cthulhu section.
However, the real show stopper is the chart in the back for the creation of random magical artifacts. I adore the idea that in a setting like this, not every magical object players may find will be useful or even something they should come within 20 feet of. Not every piece of treasure will be something they can sell, and they might even need to try to destroy some of it given how dangerous it is.
All in all, I have become a little obsessed with this book, as it has given me so many new ideas for worldbuilding of my own setting using Labyrinth Lord and the quality of this book actually directed me toward checking out the Labyrinth Lord rules. The only thing I wish I could see more of is examples of more other-worldly and wonder and horror inspiring dungeons or adventures, stuff like the cities of the white ape from Facts Concerning the Late Arthur Jermyn and His Family, or that gorgeous drawing of R'lyeh from Shadows of Yog-Sothoth. It might not even have to be something that extreme, but tips for creating realms with an uneasy feeling, maybe even a detailing of a Sea Blood village, or where the Subhumans live.
This is a setting that I would love to see more of. Medieval Cthulhu stuff exists, but actual fantasy Lovecraft? That is rare and there is simply too much unexplored territory here to be left. This book does something I wish more RPG books did and that's create a sense of mystery once you start to piece its ideas together.
[5 of 5 Stars!]