I finished it last night. I found it engaging but not engrossing. There were items when it downright dragged. I can't help but think that it is a pity Magdalen and Lydia Gwilt were not combined to make one character.
Magdalen's motivation and Lydia's drive would have made for fantastic reading, and there are far more direct looks at Lydia's thoughts and struggles than their are into Magdalen's. The latter's struggles tend to be confined to vague statements like "That night, the struggle between Good and Evil was fought once more within her," a sad contrast to Lydia's diary entries and need to use laudanum for sleep. On the other hand, Lydia's motivation was far less convincing than Magdalen's is.
The plot is less engineered than that of Armadale--until right at the end where there is an obvious wrench in order for the "right" ending to happen. A satisfying end, mostly, but definitely one where the puppeteer is apparent. It is a good book, but not as good as Moonstone or Woman in White, both of which are books I think everyone should read. This one, I think, is only for fans of the Victorian novel.