I (often) upvote every non-spam comment on my posts. Sometimes I forget, sometimes I don't feel like it. People shoudl feel entitled to an upvote :P
On other posts I seldom upvote comments. First of all, I don't always read them even if I upvote the article.
Second, quite frankly, it is a bit of a hassle, especially since you cannot batch upvote. Always wait 3 seconds...
Third there is the bad thing with the money. As long as I am not FIREd, I have to et what I can ;)