The fill flash below the main flash is a nice bonus which the Canon 580EX II does not offer. Its mainly used to fill in hard shadows where you don't want to much direct light. The Metz has the exact same hotshoe as the old 580EX and it works just fine. The USB Flash port may or may not allow the Metz to be expanded and work with new devices in the future. They have already released one Firmware update so obviously that served some use. I don't know about ratios on the fill flash but you can read the manual (search for fill-in flash) at this link to get more understanding.
http://www.metz.de/fileadmin/fm-dam/..._NL_GB_I_E.pdf
I ended up saving about $75 over what i would have paid for the Canon 580EX II Speedlite at some big name retail store.
