Fitted mine at the weekend and apart from some hassle getting the turbo to de-cat pipe tight I am very impressed.
Heard or seen people on here moan about the fit but I can say it's excellent. A mate and I did it on the floor (idiots) and it took ages, but mainly to get the standard one off - still original and 7 years old now!
The thing I noticed which may lead to them banging about is the fact that there must be at least 10mm of extra width on the hangers compared with the Audi rubbers I re-used, so the whole system can slide side to side at will, from the front mount in the middle of the car back to both rear box mounts. By the time you amplify that movement along the whole exhaust that is quite a bit on the exhaust cut out in the rear bumper.
Luckily, mine was 2nd hand and the previuos fitter had come up with an ingenious bodge. Use cable ties - i.e. the square head of one as a spacer, once you've worked out where to have it sat whip a couple of small ones in there and hey presto NO KNOCKING!
Sounds lovely, pretty quiet in resonated format but nice and deep and very nice when floored all the way up the revs. Dunno about any imporvement in power cause I de-catted at the same time so that has given it's own benefits. It also looks just right in the bumper and doesn't protrude rearwards as much as the OE system.