I cannot recommend the Xtrons Android 5.1 Head Unit highly enough, it is one of the best upgrades I've done to my car. Even mates who have no interest in cars or technology are highly impressed by it, it blows the dated RNS-E units out of the water functionality wise, it's basically an Android tablet with a built in DVD player.
One of the neatest features is that you can download the Torque Pro app (£3) and link it to the cars ECU with a OBD2 ELM327 bluetooth tool (£6) to get live readings such as Boost/Fuel Rail Pressure/EGT's/Coolant Temperature and many more.
This is where mine came from and I've had no issues with it whatsoever:
http://www.ebay.co.uk/itm/XTRONS-2-...444270?hash=item2a6cd3c4ee:g:KUMAAOSwZVlXrBnF
The quality is superb as it's got a quad core processor and the screen resolution is 1024*600. They're a fraction of the price compared to other units and offer so much more as well. Mine works with my Bose sound system, all that is required are BOSE RCA Suppressors for £20 which gets rid of the little interference at low volumes and increases the output of the rear speakers slightly. It is also possible to root them so then you can fully customise it to how you want e.g. use your own image icons and loads more etc (haven't done this myself yet but others on here have).
Have a read through this thread for more information
http://www.audi-sport.net/xf/threads/android-rns-e.248150/, ignore the odd negative comment on the first page or so as this is for older units and from different sellers. The new 5.1 quad core units are much better, the link I provided is to audio-visual-tech who have a good reputation.
