Not sure you're going to get your answer about prevalence of leaks - almost all the posts on forums will be about problems, without the numbers of roofs that are fine to compare to. Also the car hasn't been out very long to tell you what happens in 3-5 years.
As I understand, the roof is designed not to be 100% waterproof as that's too difficult in the long term, so there are some felt like seals that last better over time, and drains so that none of the water that gets through the seal gets into the car.
There have been some threads of blocked drains and some leak because of that. Proper silicone rubber seal maintenance is also discussed on other threads.
I've had no issues so far, 10k miles.
This style of roof is on a bunch of other audis and vws that may have been in the market longer, so it may be worth looking for broader panoramic roof experience as most Tig owners are less than 2 years in here?
Modified by Bevmo at 10:29 AM 4-5-2010