On using legacy flashes with the 600, check out Syl Arena who found a workaround. Since we're using wireless, you can use a cheap optical trigger to get your 580's to play with the 600. I'm debating whether to sell the 580EXII and 480 to fund a fourth 600, or just do this option.
If you have the money, I highly recommend the 600. Simple, clean, easy system, I'm really liking it.