I myself completed our 176 PR application and all other necessary things (including the professional assessment from Engineers Australia). It requires a number of papers (evidences), but everything is well written in the website and/or documents. So I didn't face any slightest challenge. Moreover, after getting my Case Officer, I get my every question answered from him within couple of days from asking. A lot many people I know from my organization have applied by their own and so far I know, everybody got their visa.
But if you are unsure, you better go to an agent.