I think the Q7 is a really ugly machine and the third row of seats is very cramped indeed, leaving no room for luggage. There is a great video of the Q7 on the "MyTouareg" website, which shows the Q7 being towed out of a muddy field in Sussex - a flat field, at that... and it got stuck.
I'm very (and I mean VERY!) pleased with my VW Touareg 3.0l V6 TDi and this engine is shared with the Q7. It is an amazing tow-car with oodles of power and the weight provides fantastic stability towing my twin-axle Senator Louisiana.
The new Mercedes R Class is one exceptionally nice vehicle and will also make a great tow-car. I'll be looking more closely at it when it comes time to change my Touareg.