We have used an agent; Ian Harrop and it has been a real help. My husband is self employed plumber and so there has been a lot of documentation and information required for his employment. it's been a real weight off my mind just having to find documents and scan them over and not have to worry about the endless list of things (much like Aimee2402). it has just been really helpful to have someone who's job it is to tell us when to do what, when to go for what assessment, what documents to send over and when to lodge different applications and after 6 months we have finally submitted our visa application tonight. (slight delay just due to renewing of passports).
Good luck with your application!