I usually reply if there's a question (one that I haven't already answered a thousand times), and I try to say something if the commenter put a lot of effort to write a very long comment or made a specific observation on something that makes me think of a good reply. But I get thousands of one-liner comments that don't really make me think of anything, and I don't think that a comment which praises me is somehow worth more by default than a comment with criticism.