You'd need to access WiFi to download the maps but yes, it can be totally offline. It depends where you are in the world, what kind of coverage you get, but it can be used offline. I certainly use a 1520 in Europe like that.
You'd be very limited in searching by POI name - offline POI names database is quite small, so be ready to type in the address.
Oh, and if you plan to use it exclusively offline, put it into HERE maps settings after maps are downloaded. Otherwise it'll keep pinging you now and then that it can't access the network.