Had nothing but trouble with mine since I bought it new about 18 months ago, so far a new engine, and god knows how many times it has been back, still has the engine light on now with fault (too rich at idle), it goes off and then back on again. Maybe most of my issues are down to the Revo software Stage 1 and not a problem with the car, though the new engine was a hair line crack in the block on cylinder 1.
Car has never let me down though, and I still love driving it to bits, If I could really be bothered I would have the Revo map completely removed, and add the MTM map, but I am fed up with it tbh and just drive it now.
I do find it frustrating though that the S3 is to suppose to be the top performance in the range, but seems so de-tuned in stock mode. I do think Audi have issues with the engine as well which does not help matters. I think it is a little dated as well in the gadgets, but they just all work.
I love the whole concept though of the Audi S3, it suits my age, as in lack of ability now
style of driving, 4WD, and most of all the safety of the car. DSG is great, fuel consumption also great if driving properly. I do think the only slight downside to driving is the under-steer, would like it just a little more balanced than it is, though I understand why Audi make it the way it is.
If you can get a good one at the right price and find the right remap, (sorry not Revo) it will give you a lot of fun.