Hey,
So when you're looking to use a migration agent use an approved one - off the government site! That's extremely important.
Secondly, there are pros and cons and in my experience, which is ongoing, i would say this is what I have found so far:
1) A reputable migration can actually be a help when getting a job as organisations spoke to my lawyers and confirmed my visa situation, thus getting me a job - incredibly helpful in my career!
2) They're extremely thorough and look over everything, then give you the chance to look at it to confirm before sending it off to the department. This ensures everything is done correctly and swiftly, without any hiccups.
3) They have solutions to problems that arise, for example I was in the midst of having my Grad visa granted but at the same time potentially my residency, they were smart enough to wait it out and get the grad visa granted, then wait for the pending residency. Basically it was smart because it didn't mean my residency may have came through and then my grad visa would have overridden it - could you imagine that! lol
That's my input and to be bluntly honest, the only disadvantage/con is the price you pay - it is astronomical.