I have a Garmin Nuvi and it's fantastic, although buying new maps for overseas can be expensive if it's only a one-time trip (around $300). Regular updates to the Aus maps cost around $99 and can be downloaded online - you probably only need to do this every 2 years.
One advantage (for me anyway) is that you can use the Garmin MapSource program to plan routes on a PC in advance of the trip and then download the routes and Waypoints to the device. The estimated travelling times seem to be far more accurate than Google maps.
The Nuvi has nice features which you may or may not need such as Bluetooth for hands-free mobile, FM transmission to use your car radio speakers in place of the in-built speaker, MP3 pplayer and Audio books player.
Whichever make/model you get, I would strongly recommend making sure it reads street names out loud (i.e. doesn't just say "turn next left"). I would also go for a large screen (e.g. 10cm). Also check out the cost of map updates and whether updates are available for any other countries you may need to visit. If you plan to use it while walking around an unfamiliar town, check out the battery capacity - you may need 2-3 hrs.