I own my Cayenne Diesel since four months and it is a wonderful car. The engine is absolutely nice and the torque is really good. Maybe, they could still improve the sound when in idle, but overall, compared to other Diesels, this is really quiet.
The design is clearly a question of taste, but for me it was the best compared to other cars that I looked at (BMW X5, Lexus RX, Mercedes ML). Especially the interior is so much nicer and yes, there are a lot of buttons (especially with the Premium Plus Package), but you get used to it. Quality is excellent, the only critics wold go to the analog clock that stopped working after a few days and Porsche has not yet been able to fix it. This seems to be a 2013 model problem, maybe if you plan buying one, go for the compass or sport chrono option to avoid this.
MPG is up to 38, I would say 32 on average. If you drive a lot of highway, you will easily go above, city (San Francisco) can go down to 22.
The price is high, but everybody is expecting a higher resale, too. I had 3000 miles, so far, and no request for oil or anything else (some of the gasoline versions seems to require oil a lot).
I would buy it again, maybe change a little on the options (it's so nice to have that many offered, but also confusing and makes it difficult to chose the important ones). Mine is jet black with beige interior and panoramic roof. Looks awesome. Most of the options are actually priced ok, when comparing to other cars, however there are a few that are absolutely overpriced, these are wheels (19 inch is even standard on the ML) and keyless entry (double the price than Mercedes or BMW).