the nav is poor & fiddly, there are far better systems out there for less money!!
Yesterday I was test driving a Mini Cooper Works - and if you think the VW Sat Nav is fiddly, then you'd LOVE the Mini's one!
Its controlled by a little joystick and 2 buttons - which means inputting an address takes AGES! The VW one is so simple in comparison with its touch screen.
As OEM systems go, the RNS510 is a fantastic bit of kit. Yes, its overpriced from the factory, but a decent retrofit for around £550 is a bargain. It has the advantage over many other OEM systems that it has a hard drive built in - and as a result you can still play CD's since you don't need a Nav DVD in all the time
Difference between the RCD310 and 510...
I don't think there is any sound differences - but the 510 has a built in 6 CD changer, the obvious Touch Screen which looks better and makes MP3 / iPod functionality much simpler. It also has the ability to display optical parking sensors, climate control, and a reverse cam. Well worth the £380 factory upgrade if you have no need for the RNS510.