Putting in my two cents. Just got mine after my GN2 suffered the faulty sim tray of death and was out of warranty by 6 months.
Very fine phone. The biggest differences I see between the GN2 and 3 are the screen resolution, and the way if feels in hand. Other than that, very similar phones in all areas.
I have been using Android for a while now, on Nexus and Samsung devices, and I am still having trouble seeing the difference between vanilla Android and Touchwiz. My issue has more to do with ATT bloatware which I just don't use.
Added issue, the GN3 has this Lookout Security app on it which I am very on the fence about. More ATT bloat, annoying, but is the security worth it? It has about 3 days to prove itself.
I disabled every app I was not using or cared for including samsung push service which was keeping my phone awake all day and killing my battery.