I've found upvote/downvote quality almost completely uncorrelated with comment quality. There's a little correlation with completely off the wall comments being downvoted. But if you look at comments where there is actual thought and some interesting statements, they are just as likely to be downvoted if their position is unpopular as a relative shoddy and cliche comment that supports a popular position is to be upvoted.

For this reason I generally don't care about comments at all, except for when comments are negative, as it inhibits the ability to read the comments. I'd actually like to see that practice eliminated, since I find that often comment "gems" have negative karma, but simply take on an unpopular position.