Well, I just got these today. In short, they are really quite amazing!
The build is much better than the XB500. They feel much more solid, and the headband adjustment is of better quality. These are just really impressive to hold in your hand, something I thought Sony would skimp out on especially for a headphone like this. But these feel very solid, and are very well constructed. The earpads are awesomely huge, and now can fully enclose my ear, resulting in much better comfort. I just cannot believe how soft these are... They are a joy to hold and wear.
Sound wise, they sound like a cleaned up and refined XB500. I can actually hear the midrange now, and the bass is now in perfect territory. It kinda reminds me of the Pro 900, because it doesn't ruin the rest of the sound with overbearance like the XB500 does, or other bass oriented headphones. In fact, these don't have THAT much bass, it actually works quite well with the rest of the spectrum. Bass goes a bit deeper as well, while not engulfing the mids like the XB500 did. Much better all arounders IMO. But obviously, there will be many fans of the XB500, that prefer it's more bassy presentation. But the XB700 to me is superior to the XB500, and it will definitely be a keeper.
But man, these are seriously one of the best headphones I've had. I'll take them head to head against my HD598 for kicks.