I had a go in one and a TTS back to back in June (when I still had the RS5, which I have now replaced with a manual S3 saloon - long story, separate thread). Here are my thoughts which I posted in the RS3 section:
Here goes:
I had the 8v S3 for almost a year and was largely underwhelmed by it, so moved it on.
The RS3 should be a huge improvement on that so I thought I should give it a go and see if it rights the (IMO) wrongs of the S3 and would make it a worthy successor.
Looks:
The front arches are RS like, perhaps not enough, but more defined all the same.
The front bumper and grill look great.
That's where it ends though. The rest is the same or worse than the S3. I prefer the S3 rear, there is something that doesn't fit about that car and those two big gaping exhausts. Wheels, awful and no choices (although easily remedied post purchase).
This was in Nardo, which I love in pictures. In the flesh it doesn't work though - it doesn't extenuate the cars lines, and is bland, especially with the aluminium pack. If I was forced it would have to have the black pack, black roof rails and panoramic roof to break up the sea of grey.
Would likely look better in Sepang (with aluminium) or white (with black), but all in all, not RS enough - should be more flares and more thought into the rear.
It's an A3 Sportback at the end of the day, so we shouldn't expect great looks, but they could have been more racy.
Interior:
Love the Alcantara on the wheel. The rest is the same as S3.
Supersports don't cut it, not by a long shot. The ones in the TT-S I drove (also today) were much better.
Seating position is terrible, you are riding on rather than in, which when added to the lack of support from the supersports means spirited driving moves you about quite a lot and doesn't install confidence.
Hate the pop up screen.
Engine:
The star of the show. Pulls very well and a vast improvement over the S3, turbo lag reduced massively.
The exhaust on the demo was the standard and its relatively quiet. Sports exhaust an absolute must.
Handling:
Felt firmer than the S3 which I liked, but didn't feel like there was a huge amount more grip if any.
Understeer was evident at one hard corner, but not too bad.
Felt a little skittish at fast corners where you keep the accelerator on the power (similar to the S3).
Aforementioned seating position doesn't help with the feel of the car. The steering doesn't feel any more "alive" than the S3, which isn't good.
Overall:
It's undoubtedly better than the S3.
However, it's still a flawed package, that didn't give me any real feeling of excitement unfortunately. I think if I were to buy one it would last about the same length of time my S3 did for the exact same reasons.
I don't think its a worthwhile move for S3 owners, it just isn't worth the premium over one.
TTS:
I also test drove the new TTS. Now there's an entirely different prospect.
Handling is spot on (far less feel of understeer tendency across the same roads as I took the RS3 and roads I have driven the S3 many times), seating position perfect, seats better feeling than the RS3 (I'm not sure if they are hugely different or different at all), steering has a huge amount of feel and, despite being down on power on the RS3 felt more than fast enough as it was great fun to drive.
It also looks great IMO, has far better wheels and, although probably not to all tastes the interior is very fresh, makes even the S3/RS3 look dated.
It's just too impractical for me just now or it may have been worthy of running some numbers.
Got back into my car and didn't want to replace it with either (which is unusual for me), although the TT-RS will definitely be worth another look when it comes. If it drives like the TT it will be brilliant and might be worth making a compromise on the practicability issues the TTS has.
I think the TTS drives better than my car does.