I think a good consultancy can reduce the chance of someone having an interview. But I agree with Calgirl, because this reduction is not significant for most applicants.
But here is a scenario, suppose there is a hole in your life/story, or some anomaly that is hard to explain, then the consultant might know strategies for this that will reduce suspicion from CIC that your application is fraudulent, etc and hence, reduce the chance of an interview.
There was one person on this forum who could not get an Indian PCC because Nepalis cannot apply for PCC in India (perhaps under certain circumstances) even though they have lived in India. A consultant might know how to come around this or what to tell CIC.
But most people don't have any anomaly and if they have time on their hand or do not have the money for consultants, then they could fill out the applications themselves.
Honestly, for most people, I don't think it would take a long time to fill the application. The difficulty for the application forms is trying to figure out all the addresses where you lived, etc but these are information that you need to give to the consultant anyway. So if you are going to write it down neatly for him/her then you might as well type it neatly onto an application form or onto a word document. Other than that, I filled up mine pretty fast and I've lived in multiple addresses across multiple countries. As for gathering documents, appendix A is quite clear on this, at least for the Buffalo office.
I do think there are some people who suffer during the application process due to their confidence in English (no offense to anyone). The application form or instructions will say something and they will do the opposite or they will give additional material which is impossible to comprehend for a first language English speaker.
Ultimately, its the applicant's trade-off between time, money, effort, etc.
