Even though this product is a year old, and much reviewed, I felt obligated to contribute. The reasoning: I'm a 20 year long fan of the original CoC game - and HPL.
This product is expertly produced and well written. The thematic approach taken is well illustrated on the credits page: Based upon Call of Cthulhu by Sandy Petersen and Lynn Willis, Inspired by the works of H.P. Lovecraft. With *inspired* being the key phrase.
The Chaosium version is about roleplaying IN the worlds of HP Lovecraft - with that focus on an identical setting and parallel plot situations. The D20 CoC game isn't meant to replicate either the fiction or the Chaosium classic. Just like D+D 3E wasn't meant to replicate TSR AD+D (thank God(s).
In addition to a stand alone game, it is clear that part of the intent of this book was to "officiate" the use of Lovecraft's mythos in D+D. Nearly 20 years after their banishment from Deities and Demigods, the Elder horrors can return to haunt the lands of Elves and Hobb- er... Halflings.
Despite my serious worries over a level based CoC game - I will blasphem and proclaim that *system* wise - this game is superior to the original CoC.
The D20 system is far more dynamic and flexible. Target numbers are simply the best way for characters and the GM to have more control/impact over the game environment. The dicing becomes a transparent medium with which to create drama in which the characters are the stars (lucky or otherwise). Most players I currently deal with find the idea of percentile scores and fixed, chart based target numbers very restricting. It makes character individualities and base traits less important. And I see no reason to enforce that through use of a dated - though very efficient - system.
The provided flavor text constantly emphasizes Lovecraft's influence (he is quoted on nearly all the pages) and the frailty of the characters compared to fantasy/action counterparts. And that the game shouldn't be about duking it out with monsters. A noble attempt to immediately nix delusions of slugging it out with a shoggoth. But there is no way getting around an investigator with 75 hit points being able to survive multiple shotgun blasts unless they are very unlucky. One freak indigent with a baseball bat should be a real threat. The feats aren't particularly disruptive - many are non combative or only add a slight "edge". But neither are they necessary. They can easily be overlooked or adjusted as play style dictates. Psychic feats seem out of place at first, but I've had several players ask about having some slight mental or mediumistic power.
Level oriented advancement won't be a problem beyond the potential for very hit point heavy characters. This is easily remedied by simply using a d3 instead of d6 for hit point advancement - and of course in the best HPL based games - having most of the situations get worse through violence.
One thing that strikes me as a bit confusing is that there is no substantial or logical adjustment to the sanity rules. With all the emphasis of a D20 in the D20 system, the sanity loss remains exactly the same - percentile based. I still think this is the best simulation of eroding sanity in any RPG, so I'm not complaining - but it seems an odd thing to leave inorganic to the D20 system in the context of changing just about everything else. I also have personal interpretive issues with the use of skills to "first aid" a character going insane (or even characters slipping to -10 sanity instead of just losing the plot at 0). The section pertaining to insanity classification and treatments is well researched and thoroughly explained.
The handling of spells and tomes is fairly true to form. There is a bit more "D+Dism" here than elsewhere in the book, but that isn't something a good group/GM can't overcome. If you wish to make the forbidden books harsher to read and their associated magic/lore less easy to use - then go for it. Monte Cook won't have you arrested for changing his game.
The monsters are well presented, with some of the text coming right out of the original RPG. There are a few key creatures missing, which I can't imagine couldn't have been made room for. The "Mummy" template section particularly interesting - and have enjoyed some of the "variant" interpretations of the various "False Gods".
Most people who shout down the D20 versions of various games have never played them, or even synthesized the rules. Many somehow reconcile a stand against corporate superstructure by turning up their noses at D20 while wolfing down as many fast food meals as their friends are willing to buy for them during their weekly gaming forays. CoC D20 core rules are no exception. If reliance on a percentile system to create a successful horror scenario is necessary - then you should probably look into a different hobby.
Playing in the tradition of Lovecraft's horror is about setting and atmosphere. Not which dice you roll. However most players I know have come to prefer dynamic, target based systems over static percentile based games. I see no issue with using a 'different' system if that makes them happy and allows you all to enjoy the story. The fact that I have used AD+D 1st ed as dark fantasy/Lovecraft is testament to the mutability of Cthulhu and the mythos. After reading the rules and supplementary text I am confident that any alterations won't affect the ability of a good GM and players to enjoy Lovecraft's vision. Bad GMs and players will always pervert a game's intent and atmosphere with a predisposition towards superpowers and mega-damage. Does this game - through use of its level/feat/class based groundwork *promote* sessions in which Investigators attack cultist hide-outs A-Team style? No - bad players and worse Gamemasters promote that. Rules are static suggestions drawn from a page in a book.