I have finally got my hands on the 580ex, first impresions are,
Build quality is as you would expect from canon, although Im not sure how long the plastic mount will last as the unit itself is quite large. (The same as my 350d body but two to three inches longer. )
Although with a 17-85mm lens and battery grip, attaching the 580ex balances up the camera nicely.
I havent really had much time to experiment or play with the flash as yet but, from what I have done so far Im more than impressed.
The recycle time for the flash is very quick and the flash has held the charge over night being instantly ready to use the next day.
Two small niggles I have with it are,
the battery door is a bit awkward to open, I think with practice it will be ok, I cant help feeling Im going to snap it off though.
and the protective case it comes with has no belt loop, these are both really really small criticisms on what so far appears to be a great flash and I cant wait to start experimenting with it!