There are a lot of things I could say about this book that would all pretty much boil down to: JUST GO READ BECAUSE IT'S SO GOOD.
From the writing to the plot and characters, White does an excellent job of portraying the brutality of Vlad the Impaler as a woman without cheapening either the character or the historical figure, since Lada's ruthlessness is complimented and held in check by Radu's sweeter but much more calculating temperament.
I'm very glad I got to see Lada and Radu through their childhood because their characters would've been much harder to stomach had they just been introduced at 16 as ruthless/brutal and calculating, respectively.
I've dwelt quite a bit on the characters because And I Darken is chiefly character-driven, with large section of "inactivity" punctuated by bursts of action. This brings me to my one complaint about the book (well it depends on how you look at it). I was expecting a lot more action from it, instead this first book was setting things up, with the second one being (presumably) where the plot really takes off.
(If anything, this only serves to make me all the more excited for the second book).