Like samurai?
Like vicious backstabbing and war?
Like demons, angered gods and magic?
Well, keep looking. This book sounds all the right notes, but is deaf to the music.
Apparenly, Kara Dalkey modeled Genpei on ancient Japanese books like Genji Monogatari, and therein lies the chief problem. Those old books are fantastic for historical reference, but in terms of entertainment value, they style is completely unsuitable to a fantasy novel. Dalkey's mistake is in believing that her fantasy chronicle is equal to thousand-year-old tomes -- it is not.
Genpei is a dry and ponderous affair that feels twice as long as it actually is, modelled (as it appears to be) on academic translations of Genpei-era chronicles. Why Dalkey thought this was a wise choice for a prose style, I have no idea; the writing is capable but dry as dust, lacking any particular voice and any trace of flourish. The dialogue is utterly formal and devoid of wit. The method of storytelling, much like a historical chronicle, seems to be 'tell, don't show'; the story tracks decades in the lives of the Taira and Minamoto families, and much of the story is devoted to playing catch-up by tediously summarizing what happened in the timespan between chapters.
It's hard to work up sympathy for any of the characters, as they all seem to be an array of scheming and politicking old men, mostly so similar that it's difficult to separate them -- and not just because of their similar and ever-changing names, as per Japanese customs. It is only possible to recognise the chief villain because he is just slightly more underhanded and scheming than all the rest of the generally despicable characters. I do not believe this is some kind of arch commentary on the degenerate, ruthless nature of Heian court politics -- though with its multiple backstabbings and demonic pacts, it really ought to be.
A fantasy based on the Genpei wars could have, should have, been a fantastic read. It was a fascinating period in Japanese history, a time when courtly peace began its decay back into bickering feudalism. The surviving writings from this time are superior to this fictional appropriation of the setting. Genji Monogatari featured a wealth of psychological realism to its characters, Heike Monogatari is a blunt, realistic war chronicle, and the mythological texts of the period have a haunting beauty that shines through their often clunky translations. Genpei lacks in all areas.
It's unfortunate that any English writer who takes on the daunting challenge of writing about Japanese history always reverts to stiff, bone-dry stylistic formality in doing so (see also the tremendously overrated Lian Hearn). Writers like these apparently want us to believe that sympathetic characters, gripping narrative and rich dialogue are purely Western inventions, unsuitable for application to the stiffly-subtitled samurai. It's not so. If you want to be entertained while reading about feudal Japan, you may as well go to actual translated sources such as Heike Monogatari; at least the events it describes actually occurred.
By all means, read Dalkey's Genpei if your fascination with samurai exhausts the fairly limited subgenre of samurai fantasy, but it really should not be first on your list. If you want a samurai drama with heart, humour and historical accuracy, there is no better than Eiji Yoshikawa's Musashi series, which benefits from a breezy translation and cultural insights that we non-Japanese cannot provide. It may not have demons and dragons, but it is substantially better than anything we Westerners have yet contributed to this uniquely Japanese genre.