Just saw your post.
I had one for a day last November, I drove it 150 miles over the course of the day on a mix of A, B and motorway routes. Once on the motorway in the cruise at constant speed it's no different to a petrol engine only car so there is no real advantage in fuel consumption in fact it's probably worse as the petrol engine has to work harder due to the extra weight. You can get it to charge the batteries on the move but that does result in higher fuel consumption. Once in stop start driving that's where the hybrid advantage comes in and the electric motor does all the work however the range is very limited, I found with a full charge the most it could go on pure electric mode was about 12 to 15 miles. Yes you can get up to about 75mph using the electric motor but that won't last long before the batteries give out. There is also the combined power mode so flooring the accelerator will use both motors to achieve maximum acceleration. I've never driven a Cayenne before so I cannot make a comparison but it did feel very heavy and lumbering a bit when driving briskly along a B road. If you do start off in electric mode with a cold engine and on a motorway at 70 I'm not sure the idea of the petrol engine cutting in when the batteries run out is a great idea given the engine is cold and not warmed up. As for the optimistic fuel consumption figures they really are nonsense, I achieved 27 mpg but I reckon on the motorway at 70 over a longer distance it would be less, I would also be concerned that as the batteries age the electric range will decrease and starting at around 12-15 when new means a lot less as the batteries age.
In summary, nice idea but I don't think the technology is mature enough to make it a viable replacement until the electric only range is up around 50 miles.
I also think the fuel consumption figures are very misleading and bear absolutely no resemblance to real world driving, this is the case with all cars but the hybrid figures are way more optimistic than anything else.
