I discovered this series a couple of months ago and binge-watched it over the course of a few weeks. It was stunningly well done. It manages to present a very rich depiction of human life - tragedy, love, violence, laughter, conflict, deception, misunderstanding - it's all there. It's really one of those rare dramas that hits all of those notes so skilfully that almost anyone can find something in it to draw them in.
It's thematically fascinating as well. The core conflict revolves around the competition between Ross Poldark and George Warleggen, who are really mirror images of one another. Ross is from a noble background, but struggles financially (he's really much better at fighting than making money - appropriate for a descendent of warrior aristocrats). He generally despises the foppishness and selfishness of the nobility, and prefers the honest, simple company of commoners. As a result, he's beloved of the villagers and workers, as he acts as their champion, but causes problems for himself amongst his peers due to his flouting of convention.
On the other hand, George comes from humble origins, but has become quite wealthy thanks to savvy investment practices. He's ashamed of his family background, which he projects onto the lower classes, whom he treats atrociously and with open contempt. Instead, he reveres the nobility and desperately wants to join them; he'll do almost anything to climb the ranks of society. As a result, while he's much better at navigating the political and economic intrigues of the upper class, he causes plenty of his own problems due to his alienation of the local villagers, who strike, riot, steal his property, and so on, largely in retaliation for his ill treatment of them.
While George is obviously the antagonist of the story, he's far from evil. His role is really more as Ross' foil: since the two are mirror images, George's strengths and weaknesses, successes and mistakes, illuminate Ross'. Just as George creates most of his own problems due to his treatment of the working class (a bias that emerges from his own shame over his working class origin), Ross creates essentially all of his own problems by pointlessly annoying the upper class.
This fundamental theme of class conflict, class transgression, and class reconciliation is also expressed in the romantic relationships, which play a significant role in the show. Most of the relationships are across class boundaries: Just because class boundaries are being crossed doesn't mean that these relationships are always good or easy, of course. The difficulties created by cross-class marriages and love affairs are a major driver of conflict, and whether or not the relationship ends up working is never certain. The show also avoids anything so simplistic as 'rich people bad, poor people good': there are good and bad people at every level.
I pretty much fell in love with Demelza. At one point in the second season (people who have watched the show will probably suspect which part I mean) I found myself yelling at Ross, "No! You fool! What are you doing!?" I couldn't help myself; I was actually angry. It's pretty rare that a show is so emotionally engaging that I end up caring for more or less all of its characters. Overall I'd say this is one of the best series I've seen in a long time.