The majority of the work involved in an application is getting together the documentary evidence to prove everything you've claimed points for in the EOI. You have to do this yourself regardless of whether you DIY or use an agent.
I think most people are of the opinion that if your application is relatively straightforward then you may want to DIY and save money. However if it's likely to be complex or involve waivers of varying types then an immigration advisor can be very useful. In our case I think the lawyer was worth every penny but that's more because I'm not sure we'd have got PR without him.
This forum is a great source of prods in the right direction and really, the SMC route isn't a tricky one to DIY if you have the points & proof of them.