There have been a bunch of topics on this if you do a search.
Personally I am not going to use a consultant. Too much $$$ and I have heard enough horror stories, which may be because some sell you their services by convincing you they can get you into your dream school even if you have no chance. I think a fair number of people who use consultants have unrealistic goals with where they are applying. Though I have seen posts on boards that some of the better consultants will bluntly tell you not to bother with certain schools and not to waste your money.
If you feel you are a strong candidate (well above the averages with a solid profile) for a school then the benefit is probably not nearly worth the cost...if you are a borderline or slightly subpar candidate then it may help and make your more confident. If you aren't a native speaker it also could benefit with writing the essays so they make more sense to whoever reads it.
I would recommend contacting them and seeing when they recommend starting with their service. If you are definitely planning on using one then I would try and get it set up now to beat the rush of people in sept and oct.