Everybody has their own opinion. I installed a RCD510 after buying it off Ebay. I don't need a dedicated sat-nav everyday. I have one running Primo, Tom Tom or Igo8 that I move from my car to the wife's.. I also use Google Navigation on my HTC Desire if I need to.
Also the RCD510 can handle a bigger SD card than the RNS for my MP3's.
It all depends if you need the sat-nav on a regular basis.
The other reasons I got one was for the improved sound quality over the RCD 300 and to keep with the OEM looks. (I know that the after-market boys will say that you can get better sound quality elsewhere.

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