I've done it for the one experience I had that I was particularly displeased about. I didn't just go leave a wordless low rating or go on an angry rant; I described in detail exactly what I was displeased with. There was no clear violation of ethics in my case, but the counsellor seemed inept, bumbling, rigid, pushy, and did not seem to respect his clients (I got this impression both from the way he spoke to me and the way he spoke to and about others).

I don't regret it and I wish more people would do it. I realize not every complaint is the therapist's fault, but I would like it if people put their stories out there so potential clients can see them and decide for themselves if they'd like to work with this person or not.

I really don't think it's all that different from rating anything else: yeah, sometimes people will have unrealistic expectations or get irrationally angry, but other times there are legitimate problems and it's worth getting the word out. If it's okay to leave a poor review for a bad restaurant experience, where the worst thing that can happen is you won't have a very good meal, then it's worth doing so for a bad therapy experience, where the potential for harm is much greater than that.