As already mentioned, the MMI system in the A8 doesn't have a touch-screen and it's when you use the satellite navigation that you realise how useful a touch-screen is. That said, Audi has done its best to make the menu navigation and text input as simple and intuitive as possible.
Given that the main control is a dial, Audi has made sure that the text/number input screens lay everything out in a circle. It's therefore easy to spin the dial around to the desired letter/number. OK, so it's still not as quick and simple as just stabbing your finger at the screen, but it's better than many systems I've encountered.
Full postcode support is off the menu, with the A8 only recognising the first five digits. This will get you pretty close to your desired destination, but you'll probably still need to know the house number to zero in on your target. There are some niggles with the postcode support though, when I entered the E11 ( that's East London where I grew up) it completely flummoxed the system and it refused to let me even narrow the search down to street name.
As with the RNS 510 in the VW Scirocco, you can also tell the system to navigate to a particular point on the map. However, this isn't quite so easy with the A8 due to the lack of touch-screen. What I realised at this point is that the button at the centre of the navigation dial is actually a very small joystick. The joystick is surprisingly easy to use once you get used to applying the correct amount of pressure, although I can't help but feel that making the whole dial a joystick as well (like the one in the Renault Laguna Coupe) would be a more intuitive solution.
Once you have the destination dialled in though, the navigation system comes into its own. You can configure the MMI to return three routes for you to choose from, as seen in the VW Scirocco. This is particularly useful, since it allows you to take the shortest route, the quickest route or perhaps the scenic route. With many systems you can tell it to choose the quickest route and you will be presented with a route that's far longer than necessary because the system estimates that you'll get to your destination a few minutes quicker. Having the three different routes displayed together means that you can make an informed decision - you're probably going to choose the shortest route if there's little difference in time compared to a longer route via motorways.
The 3D map view is beautifully rendered and it's very easy to see all the icons and points of interest are you're driving. If you prefer a top down map view, you can switch to that, but I doubt that many will turn off the 3D view. Another nice touch is that your next three turns are displayed on the left side of the screen, giving you lots of notice as to what's coming up. Of course the next turn is also shown in the dashboard display, which for me is one of the major advantages of an integrated sat-nav over an after market unit.
You get built-in TMC so your route can be dynamically altered to avoid traffic snarl ups without the need for any intervention from the driver. You can also view all the latest TMC traffic reports before setting off on your journey to get an idea of where the hold ups may be. There's also a decent selection of Points of Interest from petrol garages to restaurants to hotels. And of course you can tell it to plot a course to the nearest Audi service centre.