While I have a 16gb WiFi iPad, I didn't realize how much I would want GPS on such a large device until I tried it. I LOVE my many GPS apps on the iPhone. I use Navigon and Waze daily, and occasionally dabble with Skobbler and now NavFree.
After messing around with Waze and Navigon on my iPad, I am sold. I wouldn't run out and buy a huge iPad dock for my car, but the iPad makes an amazing passenger seat nav tool. It is also great for pre-trip planning since you can see so much more on the screen. The extra space would be pretty cool too, at least for Navigon. I love it, but that thing is a monster. I might not use the 3G a ton, but I would use the GPS a lot.
Good luck to all and Merry Christmas!