My 2 cents.
I went through 2 migration agents. The first one was for the skills assessment 2 years ago. I would say that they helped me a lot but I am still the one who processed my documents and did most of the footwork. Basically, they checked my documents and ensured that I had the necessary papers. They guided me on how to get the correct COE from my employers and helped me with the skills assessment stage.
(I only paid for the skills assessment part since it was unsure if there was a state that would sponsor my skills.
The second migration agent was for the second part, the EOI and PR visa application stage. At this point, I think it would have been better if I had done the whole thing myself as it seemed like they were holding me back.
For example, the medical certificate and police clearance. They told me not to get it until the CO asked for it. If I had done that, it would have delayed my application for another couple of months. As it is, I still did the police clearance and medical certificate plus other documents that they did not ask for and uploaded it myself. Thus, the CO no longer asked me for more, instead, gave me the grant, directly.
So there are pros and cons in getting migration agents. If you feel unsure and and you have the spare money, then I say go for it, but still listen to advice from people in this forum. If you need to borrow money to use a migration agent, then I say, skip it and process it yourself. You will need all the money you can save for your application process.
Hope this helps.