I don't use it. I find I can ignore things I want to avoid without it.
I have two basic reasons.
1. It's kind of final. I can't be sure that I will never find anything interesting in a given person's posts.
2. It's frustrating to be ignored. As sometimes used, it's like a person making a short, inappropriate response then turning their back and refusing to respond to anything I say. If you disagree with what I say, tell me. I'll be happy to respond and we can discuss it. If you think I'm an idiot, tell me why you think so. I welcome the opportunity to change if I think you are correct. If you simply don't want to continue discussing something with me, say so. I can accept that.