Many apply on their own and succeed. However, about 8 out of 9 applicants to a top-10 school are rejected. Once you have been rejected you are a "reapplicant" and it is harder to get in anywhere.

If you do have at least a few hundred dollars to spend on getting into a better school, you should be aware that the long-term career rewards of attending a good business school dramatically outweigh whatever you invest in this process, and will repay you many thousands of times.
 
Everybody knows this. That's why the competition is so tough. There are no figures, but anecdotal evidence suggest that more than 50% of applicants to top programs use admissions professional advice and essay editing in one form or another. (About 75% use professional Gmat prep assistance.)

Like it or not, you are up against applicants who buy serious help.

But that�s how it is in business � it�s not always fair. Winning goes the company that can pay for better advice. We advise you to do what you need to do to win. Don�t wait until you are 50 to invest in your lifetime earning capacity. By then it will be too late.

Below is an ROI advisory on payback from getting into a top business school.
Source: Forbes "B-Schools: The Payback" 9/25/03.