Wow. what a great idea. Im still reading it - but its as if Terminator and Yog-Sothoth had a baby. An absolute antidote to those years of Call of Cthulhu, where you'd have a few beers at a speakeasy then go out to become a gibbering insane mess. While there's nothing wrong with that classic style of play, here to some degree the odds are changed in your favor. Sure you are fighting against cultists, monsters and a choice of great old one, but finally you have a chance to triumph. The future is dominated by the horrors of the mythos, by acting in the past you can change that outcome.
The book itself? Snappy narrative. The FATE mechanics are presented in condensed form so its really a complete game (but get the core book anyway - its a great and useful purchase which will provide much more detail). Some nice mechanics on how the flow of the future is changed (or not). There is plenty of meaty background material (organized by great old one), where a specific entity will/has been released, and needless to say, at first blush it seems like your work is cut out for you.
Bottom line. A nice take on a mythos that has been applied to every genre in the RPG world. This is a game where instead of watching your SAN drain away as you behold a gibbering blob of slime, offset it by throwing a grenade into its slavering jaws. As Conan would say (Howard used the Mythos as well) "if it bleeds it can die," so very, very true.
A five star purchase. Nice work!
[5 of 5 Stars!]