toffee:
Based on my experience with GPS on Android and iPad 2 and iPod, this is what I can tell you I have found:
1. Pre-cached maps - Google Maps for Android does this far better. You can cache literally whole areas to your phone's memory so you don't have to worry about being outside of coverage areas for either WiFi or 3G/4G capable devices. I have NOT found this feature on Apple products running Google Maps. Since Apple is going with its own navigation maps this fall, perhaps they will include the equivalent. Their may be app store products that pre-cache maps, but I have not used them or searched for them. That would certainly be a primary selling point for an Apple app.
2. I have not found an iPad that can be in the middle of nowhere, have the GPS turned on, and bring maps up. I'm thinking at this point you need a GPS navigation product that has pre-cached maps built in, such as a Garmin product or similar.
3. Add-on iPad/iPod GPS devices such as those made by Emprum plug into the docking port of the iPad/iPod and give you great GPS. They will, if an internet connection or 3G/4G signal is available, allow you to get A-GPS data to make your GPS more accurate. That is not the same as a pre-cache of full map data, however. Typically the A-GPS data is updated every 10 days or so. People who use the iPad for flying prefer external GPS since those chips/units are usually more accurate, have better antennas, and "refresh" the GPS signal data more quickly.
4. It's really ashame that Apple did not use a seperate GPS chip, since virtually all Android tablets have this and they function just fine without having 3G/4G chips. Some of the newer tablets even include chips to receive Russian Glonass GPS signals. Given that Android tablets are cheaper than Apple tablets, and have more competition, you can bet that the A-GPS chips don't cost that much.