I never liked Duncan as a person, but I do like him as a character.
In The Calling, he was a thief and murderer, one who felt some regret over killing a man he'd only meant to rob but who was entirely willing to murder again if ordered to do so. He was smart, and had empathy, compassion, loyalty, and considerable personal bravery. Those are all good ingredients for a fictional hero. In DA:O we know from Alistair that his men loved him. He treats all of his potential recruits with respect (including the HN, though he is forcing the HN to join). He's even respectful to Jory right up until the point he kills him, which goes back to his character from The Calling. He's an almost perfect Grey Warden. He's smart, controlled, deadly, and will do whatever it takes.
That said, as a person, no, I wouldn't want to be around him. He might decide to rob me because he's curious about my stuff or because he thinks it might come in handy, and to kill me if I resist. He'd have no respect for privacy or personal property. He'd be friendly and soft spoken and kill pretty much anyone if he thought it was the best way to deal with the situation. He's a Player Character. Player Characters are sociopaths.