We did it ourselves for both our 475 state sponsored and our 887 perm residency and found it all very straight forward. In fact friends of our were due to apply for their PR at the same time as us and used an agent and it took them over 6 weeks to get the grant and the agent asked for lots of unnecessary substantiation. Our application to grant time (all done online) was less than a week!
So go figure what my thoughts are on agents.