In terms of accuracy to the novels, I can only comment as far as the first half of the TV series is concerned, as it seems to cover both Ross Poldark and Demezla, and I haven't yet read the second novel in the series.
The casting didn't exactly fit the characters' appearances as described in the novels – Ross' eyes and Elizabeth's hair were the wrong colour, and I think Eleanor Tomlinson is far too naturally pretty for Demelza - it looked like they had to try quite hard to make her unattractive for the first few episodes. However, the actors played the parts incredibly well, and I soon forgot about the physical differences. I also really liked the fact that the supporting cast playing the poor villagers and the miners actually looked unhealthy, dirty and poor, including Demelza to begin with.
It felt to me as though they'd lost much of the moral ambiguity of the novels, and made more of a 'heroes against villains' feeling with Ross and Demelza on one side and the Warleggan family on the other. In the first novel at least, both Ross and Demelza (who are point-of-view characters) are, although well-meaning, clearly flawed individuals, and George Warleggan is portrayed as nothing worse than a rather selfish, bourgeois rising businessman. The subtle dynamics of Elizabeth and Demelza's relationship were portrayed well, however, as was Ross' incessant brooding (Aidan Turner really does have a fantastic brooding face).
The Cornish scenery was beautifully shot and lent a stunning backdrop to the story, and the house interiors felt solid, dark and lived-in. The soundtrack was also beautiful and evocative. Overall, a surprisingly faithful adaptation of a poignantly human story. Well worth watching if you like period dramas.