I have the international quad core version. Got it from Expansys.
It's working a lot better than SGS2, and I think TouchWiz has improved quite a bit (though I still ended up using Nova Launcher), it's generally quite smooth, but there still redraw of the home screen at times when you stay in an app for too long.
Software features such as the motions such as landscape rotate to camera and pop-out videos I find to be really useful, especially when multitasking. The Stay Smart feature is also quite nice, and works very well so far.
The pentile screen that everyone is making a fuss about isn't that big of a deal, unless I keep the phone about 1-2 inch away from my face, only then I can tell. Although One X screen is a bit clearer, I don't see a huge difference. SGS3 screen for me, actually produces a whiter white than my SGS2 screen, which had a more blueish-white.
The battery life in general lasts me throughout the day, with general usage, I get around 23-30% left when I get back home and plugged in at 10pm (start the day around 8am). Everything is stock, no ROM yet, but rooted.