The seat is extremely firm and very well bolstered. It also has an extendable thi support that really makes the difference (I extend mine outward one detent (it has three). The side bolsters and lumbar are inflatable and a lot smoother and better shaped than those in more recent C Class seats, though the 2005 again was just as good or better, though non adjustable for the side bolsters.
Ride matches the seats, very firm, but compliant. I prefer a stiff ride so long as it isn't punishing, and Cadillac has about the best balance in this car of anything I've ever owned. Recent Mercedes C Class is a bit smoother, but the ATS has sharper handling and steering.
Brakes are outstanding, with Brembo by Cadillac branded brakes up front and smaller disc brakes in the rear. I took a very twisty mountain road Thursday night and drove quite aggressively, but the brakes never faded, never squealed and always scrubbed off speed with serious authority. These brakes are far better than anything on any car I've ever owned, and equal to those on my friend's Porsche Cayman.
The interior styling is just beautiful. It has many traditional Cadillac styling cues with the slender vertical tail lights especially appealing, but everything is tidy, compact and modern. This is a Cadillac, but a youthful, enthusiast Cadillac. I love the way this car looks inside and out.
The CUE system is perhaps my only complaint. This system is totally touch-screen based which is very nice, pretty and functional, so long as the car is parked or you are in the passenger seat. It is awkward to use while driving, but fortunately most functions can be accessed by the steering wheel controls and the phone's mute button also works to mute the stereo. I liked the Comand System in the Mercedes better, but once you spend a bit of time and learn the steering wheel controls and voice activated access the system is less annoying, though still not idea. I would much prefer old fashioned knobs and buttons, but it isn't a deal breaker.
The instruments are okay, nothing incredible. There is a three-section LCD at the bottom that is extremely useful, with each of the three sections configurable for anything from trip computer to navigation, fuel economy or diagnostics. The only real complaint I have is that the tachometer is pushed to the far left, and in aggressive driving can be obscured by the steering wheel or drivers hands. This would be a bigger problem in a manual transmission car, but on my automatic it isn't too bad.
The back seat is small, quite a bit tighter than my last C Class. I'm 5'9" and like to sit a bit further back than normal for my height when driving. I can still fit in the backseat behind my driving position, but foot and knee room is minimal. I could ride back there for an hour or three, but a longer trip wouldn't be comfortable. The backseat itself is terrific though, with great lower back and thigh support and the same firm padding as the front seats.
The interior is also very well trimmed in very nice leather, thick carpeting and real metal (aluminum in my car), wood or carbon fiber. Panel gaps are even and everything feels very well put together. The window sticker advertises a hand cut and sewn interior, and I believe it, with quality at least equal to my last 2012 Mercedes and better than the 2009 or 2005 that I owned previously, or my wife's Lexus for that matter.
Fuel economy is a bit disappointing so far. I've put just over 1,000 miles on the car in the week I've owned it and my last long drive (270 miles) returned 27 MPG. It was mostly highway driving at 80 to 85 MPH, but the C250 would return 30MPG on that same drive. Maybe it I'll improve as the car breaks in. City driving so far gives me just under 20MPG, where the C250 woud give 23. This car has only a 6 speed automatic compared to Mercedes' 7 speed, plus the car has a good 70 more horsepower, so I can't complain.
It's early days yet with the ATS, but so far, I'm impressed.