ok I was home with my sick daughter yesterday and while i was playing with my new softbox, she seemed curious what it was and was asking to come up so i was able to get a couple quick shots using it. Granted, i am still pretty new to understanding flash photography but i thought they turned out ok (ignore the poor composition with me cutting off my head.....i was using my wireless remote and i found it to be rather difficult to judge where things line up when i'm not BEHIND the camera

this was using my 430ex ii speedlight on manual using about 1/4 power . I believe i had the softbox about 3-4 ft from us too but i did test it further back w/ higher power and seems to do a decent job further away too.
definately seems to do a good job so if you have the 580 flash you'd be able to bump up to the Large size easily i'd think! thanks for all the help on this.