For normal height ceilings, Fong recommends leaving the dome in. For extra high ceilings, he recommends taking it out because it does cut down on the light. You'll note that the flash head is swiveled sideways. The reason for this is so you can quickly swivel it down and turn the camera for portrait position shots. I had read that the Lightshere was heavy. It is not. Both pieces are made from flexible material and they're very easy to put into place.
Using this combo, I believe, beats sliding out the catchlight reflector built into the 580ex which requires pulling out the diffuser disk (which by the way he recommends you put over the flash to help disperse the light further),pulling out the white catchlight piece and then pushing the diffuser disc back. That catchlight plastic looks very vulnerable.
This combo comes with a helpful instructional DVD, which also has some promotional material for Fong's other material. Here's his site: http://store.garyfonginc.com/liiido.html