It's not a tt but I've had my 2004 A3 2.0 tdi DSG for over five years now, I can honestly say I would not go without another Audi diesel if I sold mine. It is now on 155k miles and I will list all faults with it since I have owned it since 88K, flywheel and turbo changed (£1200)about 5k ago, radiator fans (£250) at around 100k, throttle body inlet flap (£250) 89k and just general servicing.
Considering the mileage and age of the car I would say that it is very reliable and it is remapped on a long run I can average 55-60 mpg.
Pottering around town I average 40-42 mpg. I would also say build quality is very good compared to your French cars or vauxhall sand fords the seats ect all seem very hard wearing. Also audi diesels depreciate very slowly compared to other makes.