I recently bought a 580EX II to complement my existing 580EX and I've been experimenting with off camera flash and also a home made milk jug and aluminium foil diffuser. I am quite pleased with the results. I love the way I can control both flash units from the back of the camera (40D), with no cryptic combinations of button presses or anonymous custom function that require the manual to look up and remind me what they do.
Now that I'm experimenting it is great to see the way I can control the background lighting vs the flash. Here's an example of a shot today, capturing a robin in open shade. I've purposely dimmed the background because it is not very interesting, and lit the robin mostly with the diffused flash from my milk jug and just a little from the on-camera flash to fill in shadows behind him. Some may not like the effect, but for me, as a flash beginner, I'm quite pleased.
Here's the setup and the result. The only editing was a little sharpening and a quick clone to remove the double catchlight in Lightroom. Nothing was done to levels/curves etc..


