Loupey, aside from it having a flash, the benefit of the 580EX ll as the Master controller is that it has the ability to control A,B, and C group of slave units. The C group does not figure into the ratio but is rather set from the master to fire at a pre determined power. It is meant for the hair, rim or background lighting.
I found that the Canon manual that came with my 580EX ll is awful. It never mentions that when the 580EX is set to fire that it automatically defaults to group A. This means if you are using a slave unit as key you need to designate it into group B. I had been setting my key flash into group A and not understanding why I was not seeing any real diferences when adjusting my ratios. That made my Key and Fill on the same group, so no ratio adjustment would have any effect.

Loupey, if you have not looked at the link that Alison posted ( Steve fortunately made it a sticky) at the top of this forum, take a look. Look at Chuck Gardeners section on Canon mulitple Speedlites. Very informative.
Thanks for sharing your experiences with the speedlites system