hello, i used my edge 705 last year on a tour in france and had several devices before. where to start? handling the device is pretty straightforward after a short time of getting used to it. imho the 800's advantage over the 705 is the touch screen (very hip nowadays) and the slightly larger screen, which might be a plus. other than that i think the 800 is just a bigger 705.
for the maps, there are a lot available in the meantime, both topographic and streetmaps. even free maps with a high degree of accuracy are around (
www.openmtbmaps.org). not sure if australia is covered sufficiently but it's worth having a look at them. i use them occasionally and think they're comparable to the store product, but about 200 euros cheaper
as they are dl'able for free.
the huge drawback of the GPS is (if you leave it on all the time to navigate, track your route, etc.) is the power consumption. you've got maybe 12 hours of riding, then you NEED to recharge/change the batteries. there are models available for standard batteries though, not necessarily garmin devices. however, i found a solution to recharging the battery on mine.. a device you can connect to a hub dynamo and use this as a power source to run the gps as on a power line. it serves as some sort of electric transformer and is called e-werk. it's available from a german manufacturer, Busch & Müller. i am sure there are others too.
in everyday use, i prefer to have at least a general map of the area (on paper) with me, as some of the route suggestions from the device lead me to, let's say, unrideable paths.. at least with a loaded touring bike. in addition a map does not use up your batteries
hope this helps a little!
enjoy the ride mate!