I have recently put one in, I got mine from ebay, got a genuine boot mounted changer. The Navi is cool, even better on mine because it has the full height computer on the dash so it shows you directions on there.
The navi is good if you get the updated CD, its got postcodes and points of interest on it.
The sound quality is very good, far better than the standard unit and on a par with my old alpine mp3 4x60w unit.
Matt.