I have a 2012 Cayenne S Hybrid that we purchased new. We now have 150,000+ miles on it. Car has been super-reliable with only 1 unscheduled repair (fuel gauge and sender relay had to be replaced) since we bought it new. That's the thing that has impressed me the most.
I would agree that the Hybrid does not handle as well as the other models because of the different weight balance issues associated with the hybrid system and battery. I also believe that the steel suspension cars corner better than the PASM setup we have, as the dealer loaners I get always impress me versus the hybrid. But the reality is, if you live somewhere that gets potholes or has "city" roads, the air suspension is a must.
I average 24mpg around town, but can regularly get 27-29mpg on drives into NYC or on I95 between NY and DC. My general highway mileage is only 21mpg because I tend to drive 75-80mph if traffic allows it. Running in Sport Mode can knock a few miles/gallon off of that. Best ever recorded trip over 50 miles was 31.6. Mileage is a big step up from my BMW X5 4.8 which only achieved 16.1mpg blended average. I think the 2016+ models should do even better as they have double the electric motor HP, which I've always thought would go a long way to keeping the car in electric mode longer.
The key to maximizing mileage is driving the car like a stick shift and using torque to accelerate instead of HP. Leave lots of distance between the car in front of you on local roads to make the most use of coasting and regenerative braking. Speaking of brakes, I have over 90,000 miles on my current set of brake pads, and the dealer told me to expect another 15k +/- out of them. That's insane. But you can really tell when the battery pack is full because the brake pedal gets soft very quickly.
My biggest complaint is the lack of a spare tire and no run flats. Get a flat tire and you have to call a tow truck. Finding someone who has your tires in stock is tough, and even more so for me because I run summer/winter setups. Usually the only tires available are all seasons. Some secondary tire makers are finally making run-flats in my size (265/50/19).
My wife has a Jeep Grand Cherokee Overland EcoDiesel, so while I have diesel experience, its really difficult to compare the powertrains because the cars are so fundamentally different. The turbo lag is certainly noticeable, much the way the Hybrid acts when it isn't in Sport Mode (with all of the extra boost and engine timing). Her diesel is same engine size (3.0L), and she gets about 5mpg better than the Porsche, but here in the Northeast, diesel is a good 10-15% more than Super Unleaded, so its almost a push on the fuel economy. I'd probably buy another hybrid before another diesel.
Good luck, you can't go wrong with either decision.