Hi, I had my VETASSESS docs certified by a migration agent. He charged me a fee of £200 but there were a lot of documents to certify. It was also useful as he provided some advice about which visa to apply for and the documents we needed to submit. Will all the points system changes it was also useful to discuss this.
I have not used an agent for the State Sponsorship application and don't intend to use one for the 176 (assuming SS is approved). I think the hard part was understanding the state sponsorship requirements, the visa process (IELTS etc) then making that initial application. I think we got a good deal for £200.