I have it and like it, but really feel that in ENWorld terms its a 3.5 star product - not brilliant, pretty good, but needed a bit more fleshing out. Love those GURPS sourcebooks though.
The funny thing is that you don't really need Cthulhu Dark Ages to run a game for Cthulhu set in the dark ages. There were not really any special rules to it and the history provided would probably be better suited if you just picked up an old Western Civ book and a specialized book on the dark ages.