We do not have any firsthand experience with it, but Overture Technologies is reporting that their Private Student Loan Marketplace (http://www.overturemarketplace.com) enabled students from 1000 different institutions in 2010 to get private student loans that were, on average, 350 basis points cheaper than the national average. That could amount to a $15K savings on an average loan balance over a 15 year repayment structure.
There are 12 lenders in the marketplace, and the average interest rate was 6.12%. An assortment of national banks, regional banks, credit unions, and state agencies are included. Of course, remember that private student loans are a last resort for funding your education, once federal loan options have been exhausted. If you are looking for the cheapest rates on student car loans, however, we are your best source!