gdgt

As I understand Assisted GPS it has two major benefits.

1) Faster time to first lock
2) Fills in missing satellite data with 'good enough' data when you are areas with dodgy GPS reception (not enough sats or urban canyons or whatever)

Neither of these affect the accuracy, just provide you with a faster and more consistent 'good enough' experience. If you lost cellular coverage your device will lose lock rather than just be less accurate about its lock when the coverage of sats gets marginal.

I have however seen a few cheap devices claiming they have aGPS when they have no actual GPS receiver. They are getting location directly from cell tower information only, and these won't work outside of cellular coverage. I always feel this is borderline lying about the specs, like people claiming that sub 100mbps services are 4G, and I don't know if the iPad falls into this category. I doubt it does. Without cellular coverage you won't have any background maps in the standard maps application, so you would need another app running for GPS lock outside coverage to be useful.

To get more than 10m accuracy you have to move to some sort of differential GPS system using two GPSs (or a correction service) as the 10m error is deliberately added to the system by the US Military to mess with hostiles using it.

TL:DR Without coverage AGPS will take longer to lock, and lose lock more often in bad conditions, but will be just as accurate when it has lock.