I do now and then, if I'm moved to. My opinions aren't too important, though, since I don't know all THAT much about the minutiae of editing (they're more from a viewer's perspective).
I've never in my life used the ratings to decide anything. O_o I go by the author's comments, or if I feel like it just downloading things on the basis of series or song to try them out (my connection's fast enough for that). I just see it as a breakdown of a particular opinion, basically. I don't notice them much (hell, if MY awful videos are getting 8's and 9's, something is desperately wrong

).
Anyhow, yes, I write a few comments if I found the vid interesting and there aren't many comments for it yet (otherwise I figure that they don't need my input). That's all.