Reliability?

I have put 17,600 miles so far, so the car is pretty new. In 2.5 years, I never had a problem that put me in trouble. The steering box needed to be replaced due to strange noises (it had a TSB), under warranty. The other things are a rough noise at cold start (the dealership says it is normal in some models due to the secondary air pump). As I said, my car is still new and all servicing was done under warranty, so no costs at all. I really like driving it and I have confidence in the car. And, it gets good mileage (26mpg city/39mpg hwy). Unfortunately, due to the low mileage and the fact that I got the maintenance plan as an extra in the deal, I cannot give you a better estimate on the running costs as I never put a dollar on it.

Mine is close to both mileage and age of the car in brunocav response. I bought mine pre owned but not from an Audi dealer.

There was an issue when I got it on the push button start with fob not wanting to be recognized. There was a missing sensor with evidence it was ripped out. I have no idea if that happened at the factory or while previously owned. There also was a recall on the seats that required replaement of the seat cover. It had something to do with the side air bag potentially not deploying. I believe that only affected some of the '15 cabs with sport seats. Otherwise no problems.

I also bought Audi Care for maintenance. That is less expensive than paying for it individually. Indivudal maintenance bills would have been more expensive than other cars I have owned, but the 12 mo/10K mile interval helps some.

I have the 2.0 Turbo engine with quattro. The trip computer indiates average mileage of 26.9 MPG. I have not reset that since I bought the car. I drive a more highway than city mix. I have run it on 87 octane gas since I bought it. 91 octane is recommended but based on previous 2 liter turbos I never felt that the slightly better mileage and performance with premium fuel warranted the extra cost. If you compare to other turbos virtually all reommend premium fuel.

2015 S3 Launch Edition with 14k miles. Bought Dec 2015 as a CPO with 9,500 miles.
So far absolutely no issues. Car sees mostly highway miles and is driven in an adult manner, but not babied.
Regarding Audi reliability in general: this is our second Audi. #1 was an '04 A6 2.7t that we bought pre-owned in '07 with 30k miles (aka one year and 20k miles of factory warranty remaining). What did fail was taken care of under warranty with no hassle. We then maintained the car on our own per Audi's recommendation and had no issues with the car after that (to 105k miles). Despite their reputation, my personal experience with Audi mechanical reliability has been nothing but positive.

I have a 2016 Audi S3 Prestige and in 10 months and 9,000 miles have yet to encounter even a hiccup. The car has bee flawless to date and I drive it relatively hard usually topping 100 mph on a special road maybe 3-4 times each week. Add to that maybe 2-3 Launch Control starts each week. My mpg given my aggressive driving falls in the 23-24 mpg range. On the interstate, cruise set at 80 mph, I get 27 mpg. What I don't like is the confusing set up for access to my iPhone's music. The Bluetooth works very well but doesn't provide onscreen access to my playlists. To get the full benefit of the MMI, you have to download the Audi app (once) on your phone and it hooks up whether BT or hard wire. That gives you access to EVERYTHING: internet radio, playlists, artists, etc. in addition to photo destinations, podcasts, etc. While understated, which I like, it is a blast to drive and it keeps you entertained. And the PoPo rarely notices.