I have a 580EX and the 550EX. The 580EX II is very similar to the 580EX in size and weight. Two major things they changed were not for the better. The wireless mode switch is gone and you have to hold down a button on the flash for it to prompt you to change wireless mode. The new hot shoe mount is a sliding clamp. I cannot use this flash on my 1DS MKII because it does not mount securely and flashes intermittently. Apparently my 1DS shoe mount has a very slight bend in it. I never noticed it before, and the 550EX or my 580EX work fine on this same camera.
Unless you need the external metering or the flash sync features of this flash, stick with the 580EX.