I've not used one myself, but I saw a very good example of how to use one effectively. It's for an on-line drop-ship supplier catalog. Apparently, the fellow who runs the company leads several user forums on eBay, and is the only service of its kind endorsed by them.
I signed up for it, and I gotta tell you, I was almost sold on the idea. I still may be. It was just that effective, and that well-written and executed. I think it's still there at mydssd.com.
Also, About.com uses them extensively, but I think they are poorly executed because they are LOADED with advertising that doesn't strike me as particularly well targetted.
Tell me what you think!