Did a whole day with the radio slaves and some really good riders, this weekend. I think I'm starting to understand how to use them well. For this photo, I had my 550EX flash, on a tripod, about 15 feet away from the subject. I metered the flash with my Gossen, metered the ambient with the camera, and set the exposure for 1/250th sec @ f/4, and ISO 400. I think it came out great. Of course, it always helps to have a good rider.
Incidentally, when I was in Las Vegas for PMA, I dropped my 550EX on the floor and it doesn't work right anymore. So I bought a new one last week. If I can get the broken one fixed, then I'll have two flashes. I'll be able to use my radio slave to trigger one and then the built-in IR on the 550s to trigger the second one. That will make my outdoor flash setup really powerful. I'm psyched to be able to start getting some fill light as well as the one key.