First be very careful about which one you choose to use. Be sure it is a reputable company by checking with the BBB and looking into how long they have been in business, etc. Afterall you are revealing all of your personal info and account numbers to them.
Also-compare the fees they are charging with other companies. It is an administrative fee and varies by company.
Lastly-be aware of the effect your participation may have on your credit score.
Consider negotiating with your lenders yourself. it can be tedious and take alot of time, but in the end you will have saved the debt consolidators fee and perhaps not damaged your credit score as much.