We used a rep for a straightforward common law outland application for a UK citizen.
The reason is we were getting bogged down by the paperwork, and both were far too worried about making mistakes to trust ourselves to do it. In hindsight that was lazy on our parts, but it is what it is.
Our rep made some mistakes:
1) the rep didn't ask for a medical up front
2) the rep never asked for a police clearance, after finding this forum, I had my spouse get one ASAP and we submitted that with his medical
3) the rep didn't ask for notarized proofs of our relationship (which it turns out we didn't need, but every case is individual
4) the rep wasn't aware that Mississauga was processing applications for London, and was advising we pull and go inland due to the clerical error
5) I did not pay the RPRF upfront, as I should have.
That being said, the rep did reassure us about our application, she helped us file a concise thorough (mostly) application with minimal amounts of proof. A solid essay 18 photos and only 8 pages of essay and photos, 5-8 pages of supplementary documents and that was it.
The peace of mind was worth it, the $3500 bill wasn't so much. I just wish I had been better educated going in.