It also depends on where you're using it. The WiFi-only iPad, like the original iPhone, does not have GPS (or A-GPS), but rather uses Skyhook, which triangulates from known WiFi base station locations sending out pings to these stations and calculating how long it takes for the signal response to work out your location.
This means it will be very accurate in an urban location with lots of WiFi base stations, and in some instances will be more accurate than satellite GPS because it'll work even when you're surrounded by tall buildings, tree cover or even underground (which degrades satellite GPS navigation.) As you head out into the country where WiFi stations are sparse, its accuracy will decrease or stop altogether.
On the other hand, the iPhone 3 and 4 series, and iPad WiFi+3G all have A-GPS, which is genuine GPS -- plus because it's "assisted", it works even better than regular GPS as it uses other cellular information to obtain your location faster, in addition to regular satellite signals. (By definition, any smartphone with GPS uses A-GPS, for obvious reasons.) Presumably these also have Skyhook, so the GPS on these devices will also work when you're underground, obstructed by tall buildings, etc.
As for loading maps, it is worthwhile to remember that even though the device has a big storage capacity (16, 32, 64 GB), its actual RAM is comparatively smaller, and thus a location-finding app using huge pre-loaded maps can cause memory issues. (The Apple OS will kill any app when memory runs low, making it look like the app has crashed.)
With something like Google Maps, however, the map data has been processed and broken down into smaller "tiles" which are easier to handle, but the tradeoff is that apart from what has been cached, the maps are not preloaded onto the device, so you'll need a good data connection to download new "tiles" as you move around.