“That is kind of you, however — kind and good–natured. I shall not forget it in a hurry. But you have more good nature and all that, than anybody living, I believe. A monstrous deal of good nature, and it is not only good nature, but you have so much, so much of everything; and then you have such — upon my soul, I do not know anybody like you.”

I don't know - to me this sounds quite sincere. He doesn't curse - he doesn't refer to himself or his possessions. He spends the whole paragraph actually talking about someone else. He can't finish a single sentence and is so completely out of his depth that Catherine never even really reaizes what he's saying (as is mentioned below). All of those indicate that to me he's being as sincere as he can be (which, naturally, is not very sincere!).