In wide open spaces the iPad GPS will work perfectly fine.
This is key. If you are in heavily forested areas, or canyons, etc., the iPad GPS won't work as well as a SirfIII GPS. Also, another benefit of external GPS is that you can put the GPS on the dash, roof, etc. and you won't have as many vehicle-body-signal-blockage issues.
That said, mapping software for an iPad is still behind Windows software...delorme, garmin, national geographic, ozi explorer, overland navigator are all Windows-based, and all
generally are better for off-road/overlanding than
most of the iPad apps. Notice the emphasis, because obviously everyone has an opinion, and some really like the iPad apps...
The drawback of PC software is actually the hardware...there isn't yet a platform that equals the iPad for size and functionality (well HP just release their slate, but we'll see)...so I'd guess sometime in the next year or so we'll see (1) better software for the iPad; and (2) better Windows-based hardware for using in your car.
Disclaimer: I have a Lenovo S10-3t touchscreen netvertible (netbook/tablet combo). I think it's great, and since I'm a Windows fan with lots of Windows software already, I bought that instead of an iPad (my wife has an iPhone 4 and begs me for an iPad every day). It is a lot slower than the iPad (in general), but has a lot more capability and so far from what I can see, is years ahead of the iPad as far as truly functional off-road/camping/overland software. Case in point: Try and download trails/waypoints from your iPad to your GPS unit...oh wait! No USB port...since I use my computer to create routes/waypoints/etc and then download them to my garmin, the iPad doesn't work for me. And I use it for fishing (marking great fishing spots)...and can't do that on an iPad (well, there may be an app for that, but I can't mark the spots in my waterproof/cheaper GPS and then save them later on my iPad)...
so, really it is highly dependent on what you are looking for and need
