I am using this flash on Canon EOS 30D. First one I bought it at Pro Photo Supply in Portland Oregon, took it home to test. This one is under exposure from one to two stop, I went back to their store and had the sell man checked and tested by him with two different one that included another brand new and demo. The result were the same. He admitted it is way under exposure. I returned the flash.
Second store is Shutter Bug, this one is a little bit better (one stop under exposure), I also tried 430EX in store to compare, believe it or not the 430EX is brighter but WB cannot compare to 580EX II.
Third store is Best Buy. This one is dead on everything and worth every penny. I used it for my brotherís wedding on October 06th, 2007.
Canon might have a bad batch to me.
Depend on your skill and how you use and utilize all of it functions. Over all, it is a great flash.