I found Sten to be highly interesting as a companion, because his perspective is so different; that said, I was never swayed by his logic.
I find the qunari as a whole to be fascinating, and mostly well written. I think the arishok in Kirkwall in DA2 was superbly well written, and acted. I think some aspects of qunari culture are valuable because of their pragmatism.
All that said, a lot of Sten's justifications, especially regarding violence, ring hollow. Simply reasoning that, "in qunari culture..." is not a valid, objective justification for the action or actions. I believe he wants to take responsibility for the murders (been a while since I played DA:O), so that's good. He doesn't ask for any mercy. But once he's part of your group, he seems to think the qun is the ultimate answer to everything, without question, and doesn't think much of anyone's rights (or that they even have rights) outside of the qun. That's fine if that's his perspective, but it does make him a little crazy by everyone else's standards.
So he's a great companion for learning about the qunari, but he is crazy by normal standards.