Experience with Short Sale Negotiations?

For the agents out there - Have you tried, or had experience hiring a short sale negotiator to help to deal with the banks? From my perspective(as an investor), this has worked well because I am using someone who has actually worked in the loss mitigation department of a major lender, & now does negotiations on a full time basis.

It seems from reading through a heap of blogs about short sales, the negotations( & sellers walking) is the most daunting task that faces realtors. Your feedback is welcome & appreciated...