Fiscal sponsorship?

Greetings: Do any of you, my dear readers, have experience with working with a nonprofit fiscal sponsor? Any experience is appreciated, but a contact with someone on the administering side would be especially welcome. And words of warning.

I have to think that fiscal sponsorship relationships would be a good opportunity both for “independent disaffiliated” organizations who want to expand their mission and for organizational entrepreneurs who don’t want to invest too much effort in getting a group started.