I have used on-line critiques in the past and they have been very useful. I had a memoir piece placed in a journal that accepted a very low percentage of admisssions after going through the critiquing process. Recently I joint another on-line critique site and it was good for the first piece I submitted, but the second one is longer and it is hard to get anyone to read it. You gain points for your critiques and noone wants to spend forever on a long piece. So I found another site and posted part of a play there, that I think can stand by itself. Before I can read any of critiques of my play I have to critique other submissions. I will see how it goes.
BTW, the first critique site I used, I forgot my password and when I request that it is sent to me I don't get a response. Either their system is messed up or I pissed someone off with a harsh critique. I don't think its the second, because I only submitted one piece and I was apprehensive about getting shredded.