I've had to start playing with XML-RPC, and it's amazing this didn't catch on sooner.
Of course, now we have SOAP web services which do the exact same thing ... but the functionality was there way before SOAP came along. Just goes to show you what a little corporate sponsorship will get ya.
Still, it's rather interesting to actually have to do everything manually ... from posting the xml-rpc structure, to parsing the xml response ... gives you a new perspective. That being said, I know you can do the same thing with a SOAP web service (after all, it is based on open internet protocols), but you wont find me doing that, no sirree. SOAP seems to me to be a whole lot more convoluted than xml-rpc, and with all of the automated tools available in modern languages (read: .NET and ColdFusion), there's really no need.