Over £1k and no HD?? Thats very expensive for something that you then have to link to your ipod afterwards. The RNS510 is linked in and charges through the cable in the armrest.. whilst all tracks and info are on screen. tbh I am really struggling to see the advantages of and aftermarket system... unless it was coming in under the cost of OEM. 
What does it offer? It does not have frequency tailering like the oem unit. What this OEM thing does is boost 5Khz and reduce the bass frequencies as you turn up the volume. (Its a saftey thing VW built in to stop numpties killing speakers and claiming off the warrenty) This makes it change the way it plays music so its crap basically. It does not have as low a powered continuous Watts built in amp so will be better at producing dynamic peaks tan the OEM, It has a flatter frequncy responce and also adjustable equaliser to help compensate for the oem speaker quality. It has much more upgrade ability pre-outs, pre-ins, video in and out, DAB add-on box ready etc etc the list goes on. The Iplop lead is simply a lead that plugs in the back of it and im sure if this was installed by a good audio dealer they could interface the MDI into it(i have not seen the wiring diagram for the MDI yet but im sure it will be easy adaption). Same with the oem reversing camera(I have looked into that one and not a problem). Full post code and iirc 3d mapping. etc etc Theres lots more on this than the blaupunkt oem unit.
The HDD do you guys ever use it with memory sticks and Iplops so easy to use?
There are also much cheaper aftermarket units available that do not have as much upgrade ability as this kenwood one.