With aGPS how much does the reception improve by? Let's say you are travelling in a larger city in the CBD with skyscrapers are you able to get a reception or do you still need a clear view of the sky? Let's say you are in a narrow street with market stalls and the like. Or if you are in a underground walk way trying to find your way around when it's very cold outside - think Japan/Korea Winter.
It can tell the GPS where the satellites are so you get a faster lock on them etc, otherwise GPS can take a few minutes sometimes to get a lock especially if you are already moving.
Then there is the use of cell towers to help get a position, not that accurate.
Then there are location services like skyhook and google that use wifi access points also (that is why google was war driving at the same time as doing their street view photos, collecting where all the wifi access points are and recording their locations). I would assume this is going to be a bit more accurate as there will be lots of wifi points etc. I haven't read too much about it though. If you have a quality sensitive GPS chip you won't be too losing signal too often even in cbd's etc.
I drove around in san francisco (downtown) and los angeles on an old garmin nuvi (260w) with no agps etc, it had no issues really though of course if you have the choice take agps :).