They have benefits and some serious drawbacks.
Benefits – They can help you with the admission process (like SOPs, Recommendation letters). They certainly make the entire application process less nerve-racking. Often, they offer statistics on student profiles and where they got admits. This is useful information. They can also hook you up with current MS students. Again, useful stuff.
Drawbacks – How do these academies advertise their performance? “100% placement guaranteed!”. Or something like that. What does that mean? How does it affect you? Some of them (not all) usually convince you to apply to lower ranked schools, where the chances of you getting an admit is almost guaranteed. Example: You might be a good candidate for schools ranked between 25-50, but he would advise you to look at schools ranked between 45-70.
My advice would be to choose your counsellor carefully as he has the potential to possibly ruin a few years of your life. You should definitely use his services for the application process, but the final decision on the schools SHOULD be yours.