The RS3 is honestly the absolute perfect car on paper. It's fast as hell, handles pretty well, runs 10's stage 2, etc. It also has one of the best engines, and definitely THE best transmission I've ever used. Absolutely fantastic car in a straight line, better by far than anything else I've owned.
Two problems hold me back from keeping the car - which I actually just traded this weekend for a new car for my wife. First and foremost is Audi. Mods are NOT advisable on this car - at ALL. From the mild - take your car in for anything at all, even a nail in the tire, and they have been instructed to scan your ECU for tunes. If they find anything, your powertrain warranty is officially blacklisted. Granted you can fight that - but it's going to be one hell of a battle, as has been proven my multiple members. Extreme examples included Audi corporate denying warranty claims for transmission shifting issues, because a user had stickier than stock tires on the car. They've gone absolutely mad since they've been paying out for dieselgate.
On the STI, I'm more than happy to take on that liability. Parts are cheap... $3500 for a good built short block if I blow the engine? I'll take on that liability NO problem. $15k for a new 5-cyl in the Audi? That's just the part. Labor is pretty damn high, as you imagine, and vendors quote in the realm of $30k for a built block, installed.
Next on the list is the overall reliability. Electronic systems are REALLY funky on these things - I constantly have electronic nanny sensors alerting, etc. There's a really well-placed IAT sensor in the intake on these engines - directly above the exhaust manifold and turbo. Sit in traffic on a summer day, and get on the gas a bit when it clears up? You're going into limp mode, a solid 50% of the time. It didn't happen to me at all last year after I got the car, but I've had it happen twice so far this year. It sees extreme IATs, and shuts shit down. Just more of Audi's brilliant engineering, honestly, the same stuff we've been seeing since we first got the S4 in the US.
Rattles! The sunroof creaks. CONSTANTLY now. People have had the entire panel replaced 2-3 times, and it still creaks, groans and rattles. Back deck rattles. Door panels rattle. I'm not OK with it in my STI - but again, I'll take on the liability in a sub-$40k car. On a car that MSRP'd over $64k? Not a chance.
Handling of the car is great - but it is NOT on par with the STI. At the end of the day, it handles like a FWD car. Push hard into the corners, regardless of your driving, and it's going to plow. There's a reason it gets left behind on C&D's lightning lap, despite how fast they are. It is NOT a confidence-inspiring car to drive in the twisties, not like the STI is.
Another electronic annoyance is the non-defeatable traction control. You can 'defeat it' by turning it off - but if you manage to get it just right and kick the ass-end out, it will turn it back on for you IMMEDIATELY, while screaming at you with a loud beep.
The car's personality changes with each turn of the key. Sometimes it feels like a rocket. Sometimes it feels slower than my wife's Golf R. It all depends on its mood at the time. It's a weird thing, and I've not heard many people complain about it, but it's definitely a thing. I've started the car up, and on my trip somewhere, it's felt lethargic as hell. Parked, did my thing, drove back 30 minutes later, and it's a monster. No rhyme or reason.
Lastly, gas mileage. I thought I got really bad gas mileage in the STI. I was wrong. My lifetime average on the RS3 was 16.4mpg. I definitely admit that I drive it hard, and I'm willing to accept that - but gas mileage is weird just like power is. I went on a ~35 mile highway drive this weekend - 23mpg average. Did the exact same drive, in the exact same conditions, the very next day? 29mpg. My commute to work - some days I drive it hard and get 16mpg. Some days I drive it hard and get 18mpg. Throw it into comfort some days and I might get 22mpg.... or, in the exact same conditions, with the exact same comfort mode driving, I might get the same 16mpg as when I beat on it. I spent a year trying to figure out if it was something I was doing on my commute, but there were zero variances, as far as I could tell - just the car decided it was more thirsty some days than others.
So, the RS3 left me craving my STI again. Something more analog, where I have a bit more control. Something a bit more rugged, something that I can mod, and something that's made to be driven all the time, instead of only on the autobahn. It's going to take me a bit to get used to how (relatively) slow the STI is, but I welcome it. The STI really is in a class of its own - you can compare it to the S3, Golf R, etc - but none of those cars really are anything like the STI, other than in the general power they produce, and that they have AWD. The STI is, out of 23 cars I've owned, the best driving experience I've had. The most confidence-inspiring, and the most connected I've ever felt to a car, save for maybe one exception on that in the MR2 Spyder.
Would I recommend the RS3 to anyone? Definitely. It was an awesome car! Just be prepared to either accept what the car is out of the box, or the liability of parts that cost literally 5-10x what the same parts on an STI cost. I'm not a 'keep it the way it comes' kind of guy, and I'm just not interested in that kind of financial liability if anything goes wrong, so... Time to go back to my Lego car, the STI.
Edit - just to throw one more thought in there - The RS3 always had, for me, an ever-present feeling of a ticking time bomb... Audi is not known for reliability in their top performance cars, so, while they have absolutely improved, it's still an issue. It's caused by the massive over-engineering of every little thing, sometimes with very little gain over the more simplistic part. It's a tough feeling, owning a car like that which can become a liability at any given moment. A bit similar with ringlands/bearings/head gaskets/etc. in the STI, with one huge difference - I can fix that stuff permanently, and still send my kid to college.