I use a Tow ball mounted carrier regularly on my D3, which is fitted with the adjustable 'fixed' towball instead of the removeable one.
We now fit three adult and one slightly smaller bike on it with ease, and it's far more stable than I ever thought it would be.
New Bike Rack
I find it very good, although you obviously can't tow anything at the same time. I understand the newer ones are lighter etc, but being a flat platform I could carry all sorts on mine.
I do however run a chain around it and the towbar mounts when we leave the car whilst off on the bikes.
There is a lot of discussion and useful information about them over on
www.disco3.co.uk
Nasher.