This is an issue that polarizes people to one of the two extremes. People either swear by admission consultants because they see great value or they swear at them because they are dissatisfied with the service they received. The truth, as with most situations in life, is in between.
A good consultant can definitely help you polish your application and can give you that extra push to succeed. But there are too many fly-by-night consultants who either have no clue about the process or have little time to pay you the level of attention you need.
My take:To start with, educate yourself thoroughly about the admissions process. Read this forum until you become sick, get the montauk book on admissions and paul's book on essay writing. Soak in all the information and master this whole application thing, atleast in theory. Then, play the field and talk to as many consultants as you can. You will be in a far better position to judge if he/she will be able to help you or not. Also, when you decide on a consultant, talk to the actual person who will be working with you and not with some marketing person.
Good luck with your apps!