I spent ages trying to decide whether to buy the 580ex II, 430ex II or the top end Sigma equivalent. In the end, I went for the 580ex II - it was expensive, but so far, worth it.
It's possibly not the lightest flash to carry around, and can make your SLR feel top heavy, but I don't often use it on camera and use it "strobist style". As for this weight issue - I'm not sure if it is that much different to other higher end flashes.
A couple of months ago, I had it on a light stand about 5-6 feet, it blew over and hit the floor (stone paving) on the back of the hinge. I can't see a mark on it, and it still works perfectly.
I wish Canon would make a wireless flash controller and not just the IR ST-25 (being infra red, it can fail in bright sunlight, and is useless if you don't have line of sight). Using a 580ex II to control other Canon flashes wirelessly makes it an expensive flash trigger.
I've noticed a few people mention the hotshoe lock not being very good - you need to make sure that you move it all the way over and that it does lock properly, so that you can't move it back without having to depress the release button. So far, I've had no problems with this.