To be honest, I've used it a few times and it seems to be when somebody has really wound me up over something. After a while though, you get sick of seeing the "this user is being ignored" or whatever it says. I don't think it works really that well, it's a virtual way of throwing a tantrum except nobody else knows you're having one! After a while, I reversed it and currently ignore nobody on the forums, at least not with the ignore function!