I have absolutely no memory of my guidance counselor. So I suspect she did no harm, but wasn't particularly helpful. Or maybe it was me; maybe I didn't let her get to know me.
Given the huge numbers of students each counselor is responsible for -- at least in large public high schools -- it seems to me that teens must take responsibility in making themselves known. Our teen dropped by the freshman counselor to say hello, as did I. This was back in the fall, and she filled me in on some basics: watch what's on Facebook, because colleges do; remind the student that there is no need to stress out (yet?); and it's never too early to think about those essays.
In sophomore year our teen gets a new counselor. I will urge our student to visit early and often. I would imagine it's far more effective to counsel someone the counselor knows rather than a total stranger.
A Public Agenda and Bill and Melinda Gates Foundation-sponsored study looked at the role of guidance counselors. It wasn't especially flattering. It is difficult to imagine when counselors will be able to provide the time and advice needed, given the grim present and future of school budgets.