So I recently went from a D3 4.2 TDI to a D4 with the same engine, my reasons for changing were mileage and age as opposed to toys but I think it is fair to say that the new car has a lot more toys and feels like a much more modern car. Navigation on the D3 always seemed "old hat", even when it was new, the D4 version works so much better and google maps is a novelty. Adaptive cruise on the D4 works really well in stop start traffic, the new gear box is just as good as the old one but with the added bonus of a improved MPG. the car is quieter than the D3 and feels a nicer place to be. The downsides are few, smaller boot being the main one.
I have been driving round in a 1 month old 3.0 TDI for a week ago while my car is having a few issues attended to by the supplying dealer, this facelift car actually has a smaller boot as it is fitted with an EU5 engine and so I suspect the AdBlue tank is in the right hand side rear wing further eating into boot space.

Also it isn't as quiet as mine, don't get me wrong still not exactly loud inside but I'm not sure if that is the glass sunroof, lack of alcantara headlining, smaller engine or I suspect a combination of them all.
Personally I like the "looks" of the D4, I'd much rather be anonymous than flashy so looking like a "big A4" is perfect to me
If you can afford it then I'd say go for it, be aware they seem to be very spec dependent, lower spec cars seem to hand around for a long time where a high spec will sell for the same money in hours, I picked mine up from a main dealer before it was advertised (they get listed on audi.co.uk before prep). Good luck with whatever you decide.