The camera works great with the 580 EX Flash ON body. However, when you start using it externally, it does not function optimally. I used my 40d for most of the shots at the last shoot for this reason. The 5dii was under-exposed except when I shot manual flash...and the problem shooting with the Canon Wireless system is that each flash must be fired at the same level in manual (unlike when you shoot in E-TTL, which allows for ratios). In order to get accurate speedlite control with the 5dii, you need to use the 580 EX II flash.

James