I've been using the Hema 4WD Maps for the past 3 years on my iPad (1st gen) and it’s worked brilliantly. A broad and extensive range of maps are on the device and you can buy others from a map shop (which I’ve never been able to do successfully!?). I also have the app on my iPhone but the small screen kills me!
I was in the Kimberley last year with the iPad and it was great to have the GPS tracking on the route just to get a handle of the “are we there yet?”. I now prefer the Hema iPad over paper maps but an issue is that you only see a small section of the region at a time depending on the level of magnification. It can be distracting reaching across and swiping around to look further afield but it has a brilliant ‘centre of screen’ function to bring you straight back to where you are. It's a big app taking up close to 2gig of memory.
I mucked around for ages before settling on the best glass-mounted holder for the iPad but got there in the end. I have my iPad powered by the deep cycle circuits so it runs all the time without fail. The app’s tracking also means if you choose to view the Australia 1M Map it shows the routes of every trip you've undertaken which is pretty cool.
I also have Alpine's satnav in the in-car unit but it's pretty hopeless off-road. And I also have the Hema Explorer app but have never been able to get the hang of how to use it. With the bunch of blokes I normally go off-road and camping with we have several different forms of electronic and hard copy navigation between us so someone always has something of assistance when needed.