Hi,
We didn't use a migration agent and it's quite simple to do yourself as long as you are very organised and methodical!
The thing we found was that everything takes a lot longer than you anticipate. Until they give you an invite then it all moved very quickly for us.
If you end up doing it yourself i wouldd suggest collating all your evidence when you submit your EOI as then it is ready when you get an invite, also make sure you can back up every point you say you have as this is a reason they refuse visa's.
We downloaded to booklet which supports the 189 visa pplication and used it as a tick list.
Also (sorry very bitty but the whole saga has just come flooding back!) when you submit your visa book your health and police checks, a lot will say wait for them to tell you to go but if you go and get this done it tends to speed the process. From lodging our application to grant was 4 weeks so it moves fairly fast if your a step ahead of them.