I can sum up Cantarella in five words: angst, angst, and more angst.
As a historical manga, Cantarella delivers, but it delivers in such a half-hearted way that I can't bring myself to recommend it. The facts are all correct, certainly, aside from the obvious references to the supernatural, but that's all they are: facts. They don't come to life, there is no vision, and often it felt like Higuri Yuu was simply ticking items off a list and regurgitating information she read in history books. Basically, it fell flat.
As a BL manga, Cantarella does not deliver. At all. The BL, as the one-sided, filled-with-betrayal kind is full of needless angst, and I've found myself wishing more than once for everyone to just die already and end my suffering mercifully. Unfortunately, this did not happen, and I had to contend with ten volumes of this ridiculous soap opera, all the while trying to brutally dismember everyone with my mind. (It did not work.)
As an interesting and unique story in and of itself... well, your mileage may certainly vary. For me, it was definitely a complete miss. The story is basically a rehashed version of Cesare Borgia's life, with supernatural elements to make it more interesting and exciting. Of course, this is not necessarily a bad thing--any premise, with the proper execution, could turn out to be spectacular.
Unfortunately, Cantarella did not have the proper execution, nor did it, in fact, have a single likable character. (OK, maybe I'm being too harsh. I did like Volpe a lot. He seemed the only sensible person in the whole thing, actually.) But, what Cantarella really fails at is character development. No one in the whole entire series ever changes. They never develop as a character. They never gain their own personality. They're basically all caricatures, cardboard cutouts with human faces.
The worst of these is probably Lucrezia, who, in the beginnings of the manga, is defined entirely by her forbidden love for her brother Cesare. After that love is over and done with, she is then completely defined by her love for Chiaro. She never gains any personality of her own. She never develops as a legitimate character. Even after ten volumes, she still remains a complete and utter plot device, meant to separate Chiaro from Cesare to create maximum angst.
Chiaro is also equally bad, as from the onset of the manga, he is described as this great "angelic" figure, and this image stays with him for the rest of the manga. (Even Lucrezia thinks he's an angel!) He's never really given much depth beyond how good and kind he is, or how necessary he is to Cesare's survival. I don't even understand how Cesare could possibly like him--he isn't a person, he's one of those storybook princes who speaks all of two lines and then gets the princess!
Personally, I think the two deserve each other. They probably get along so well because they both have absolutely no personality.
Some of the characters' motivations are a bit dubious to me as well, requiring a certain amount of suspension of belief. Even those motivations that are explained are not developed in depth, simply touched upon in passing.
The worst of these motivations is the main character Cesare's. Throughout the manga, he has mentioned that he wishes to rule Italy before his body is taken over by demons, but it is never explained precisely why.
Now, historically, Cesare Borgia was an ambitious person, but Higuri Yuu instead creates an image of innocence for Cesare (in order to make him a sympathetic character, no doubt), one who would have been kind and generous if he had not had his soul sold to the demons.
So then why does Cesare want to conquer Italy? It can't be the demons--because Cesare specifically explained that he wanted to rule Italy before the demons can rule him. It can't be for his father--Rodrigo hates and fears Cesare, and Cesare has clearly no great love for his father in turn. In fact, Cesare seems to be mainly operating outside of his father's influence, without the other's consent. So then why is he so determined for conquest? I doubt it'll ever be explained--it's not as important a plot point as, say, the angst, after all.
So, who would I recommend this manga too? Casual manga readers who have a penchant for supernatural, (w)angst, and tragic romance (which is really not all that tragic, but you know). I have to admit, though, that Cantarella is a page-turner, if only that it is an absolute trainwreck (and you know how we all delight in watching those). Otherwise, there's really nothing else I can recommend about it. There are definitely good historical, BL, romantic tragedies out there. Cantarella is not one of those.