Does anyone have experience in instituting outsourcing of the billing and collections functions, and what are the top service providers?

Every single year that I was with Allianz, we went out to market (I had sourcing/vendor management responsibility too.) for those services. The farthest I got was with Mellon Bank (College Station, Tx.), but in the end, as they would have to host our AR system via SAP, they did not want to commit. I did outsource end stage (> 90 day past due) receivables to selected law firms that specialized in that work. Sometimes all it takes to see the cash is legal letterhead. However, we worked very hard internally to vet credits, and rigourously collect what was due. We therefore made only occasional use of outsourced collectors.