Physically, it's quite appealing. A large, comfortable font with two columns (with somewhat overly large top and bottom borders to my eye, but that's a quibble), color cover, and b&w art throughout (including most, but not all, of the monster descriptions). It's purely personal preference, but I'm not a fan of the art, although I prefer it to nothing.
The "new magic' section includes 24 new spells and formulae (formulae being a new type of hybrid alchemy/magic) all of which can be added to the standard LL spell lists, and most if not all seem drawn from specific Mythos stories. This brings up a minor point of confusion I found with the rules, which I'm not sure is merely an editing problem; I can't figure out if magic-users are supposed to be included in a RoCC campaign or not. We are told:
These cold realities eliminate clerics, magic-users, and subclasses of these as class options. In this type of campaign there are only three classes that are most appropriate for inclusion from the Labyrinth Lord core rules, the fighter, magic-user, and thief.
So is the magic-user in or out? The passage seems self-contradictory.
There are obvious comparisons between this supplement and both Call of Cthulhu by Chaosium and the AD&D Deities and Demigods section on The Cthulhu Mythos. The monsters section of RoCC includes 41 monsters and singular creatures, clearly more than the Deities and Demigods book included. Again, all or at least most of the creatures are drawn from specific stories from the Mythos. Several of them can be used as player characters, for those who prefer their fighters to have "The Innsmouth Look". Ditto the 13 magic items (plus a section on creating random items, including 100 different nasty effects), and there are also rules for the effects of reading the various tomes which so often play a central role in the stories of the Mythos.
There are also new rules for psionics, and an appendix for including such in a Mutant Future game, which was nice as it makes explicit the notion that the book can be used with either game. The whole is topped off by an appendix of literary sources; I found this sort of the reverse of what I was expecting, as it gives a source for each creature, spell, and item, rather than just a list of Lovecraftian works.
On the whole, this seems to be an enjoyable and useful supplement if you're inclined to include Mythos-type elements in your fantasy campaign. To my mind the chief benefits are the practical ones; monsters, spells, and magic items. I may very well use the "reading eldritch tomes" rules in my own campaign. One rather expects rules covering this material to include mention of insanity, because such is the thrust of Call of Cthulhu and that is the 900 pound gorilla in the Lovecraftian RPG room. The decision not to include such may well have been a conscious one just to avoid that stereotype. It is certainly not a "missed requirement" so much as a "deliberately avoided expectation".
I give it a B+ overall. Definitely worth the money, and worth getting if for no other reason than it expands the medieval fantasy RPG genre in general and the LL sphere in particular. I would very much look forward to other supplements in similar vein; Dying Earth, Sword and Planet, etc.