I have the S2, and I'll definitely be grabbing this guy when my contract is up for renewal.
1) The screen. Samsung's flagships have always been amazing to look at, and one of the few things I don't love about the S2 is the fact that I can make out pixels at a short distance.
2) The camera - Without reviews, I can't know for sure, but I expect it to be a marked improvement ( 20 shot bursts, photos while videoing ) Points toward a dedicated image processor.
3) Quad core? Well, I don't expect to see a major performance improvement, ( SGS2 is underutilized as it is ), but as far as power efficiency goes, this will be noticable
4) Do I need to really have a reason? It's so very pretty, and a man must always have the latest gadgets, right?