One of the great things about live view and something that I've started to use on the 5D MarkII is being able to manual focus in available light. This is great when you have a speedlite on the camera, because it will "approximate" the exposure with flash enabled. The ISO can be set to 100, but the camera will turn up the ISO for the display only (it changes back to 100 when you actuate the shutter). So I can focus in available light on ISO 100, and then use my speedlite to actually illuminate the subject. It's very handy, even when not using a speedlite too.