Nissan will state that conservatively so as to not cut into sales of the 6MT. But the reality is that half-a-second is fairly slow and unresponsive, but to the common and uninformed person it may seem very quick. I've actually used various accelerometer tests (iPhone 3GS apps) on my old 350Z, which was a 5AT (they're virtually identical in design to the 7AT, aside from gearing) and you can see how acceleration hardly fell off with every shift throughout the entire graph.
I then compared it to a VW GTI (DCT) which showed virtually no drops in forward momentum, and then to a 2007 Lexus GS300, which showed noticeable spikes with every gear shift. Nissan really designed a superb and super fast automatic and they improved it tremendously with the 7AT, because the shift logic is actually even better and quicker.
If you watch this video towards the end you can hear the gear changes are much quicker than half-a-second.
370Z with automatic transmission Rev match explained - YouTube