The ATS is a compact sport sedan which is aimed at the BMW 3 series. That's pretty obvious. They used the previous editions as their benchmarks, as the current F30 was not available until recently, and size-wise, it's very close to the previous 3 series in interior room, which didn't seem to hurt that car's sales at all.
I had a chance to sit in a 3.6 Premium at Atlantic Station this past weekend, including with the engine running. It was Black Diamond, with the Black leather/aluminum trim, and it had the cold weather package. MSRP on that car should be about $49k, including $1000 for the paint. While that seems like a lot, I priced against a BMW 335 Sport, similarly equipped, and that car would be about $55k. The M Sport might be more equivalent, due to it's adjustable suspension, but I didn't price that yet, and that would cost even more. Bear in mind that the premium is quite loaded; to be fair, though, we should add $1000 for the sunroof that is standard on the BMW, which makes it approximately $5k more expensive, comparably equipped. The equipment is not exactly comparable, but pretty close.
So far, all of the initial tests have shown the ATS to be at least as good overall as the BMW. There have been no direct comparison tests, though I am sure they will be happening quite soon. Driving impressions show great handling and brakes with a firm but smooth ride, and good acceleration. Steering feel has been praised, and may be better than the BMW, which has been criticized for being somewhat numb. Only a back to back test will reveal the truth, however. The brakes have been widely praised, and that is another BMW weakness, as they are prone to fade, even on the M cars. The BMWs are probably a bit quicker, due to the 8-speed transmission, which I have read is coming in the ATS, but not yet available. Perhaps the closest match will be between the 2 liter turbo sticks; the ATS is rated as more powerful, but many think the BMW is way under-rated. The BMW gets great mileage, thogh the stop/start feature has been widely criticized as rough and jerky, and many will turn it off because of that. The BMW 4 has also been criticized for it's uninspiring soundtrack, while the ATS reportedly sounds great. The 3.6 may not be quite as quick as the 335, due to the relative lack of low-end torque compared to the 335, but it should be lighter, and takes regular gas, compared to all of the BMWs.
Styling-wise, I prefer the BMW and the CTS, though the ATS is handsome enough that it would not deter me from a purchase. THe interior of the ATS is quite a bit nicer than the 3 series, and more comfortable, though just slightly less roomy. The BMW is now possibly the roomiest car in it's class, and definitely roomier than previous editions. When I sat in the F30, the car was notably roomier than before, but I didn't find any of the seats to be particularly comfortable. The interior styling was an improvement overall, but not great by any means, and build quality wasn't all that great, either (the 3 was never really meant as a luxury car). As to the ATS, after I adjusted the front seat properly, I entered the back seat, and was pleasantly surprised by the room! I really expected less. Now, I'm 5'9", so I'm sure with the seat all the way back it would be tight, but that's true for every car in the class. Moreover, there was adequate headroom( a bit better than the CTS), and good foot room under the seat. There was also more foot clearance in the rear door than the CTS. Comfort, both front and rear, was great, and quite surprising. The front Sport Seats were very comfortable, and the side bolsters could be felt, though I can't tell how well they would work on the track yet; I suspect not nearly as well as the Recaros in my CTS-V. The rears were also very comfortable, especially sitting behind my seating position, and these seats were much more supportive than the rear seats of the F30. The interior was very attractive and well built, though not quite as plush as the CTS. For instance, the CTS dash and door tops are padded as well as sewn, whereas the ATS is just hand sewn. The trim in the ATS is way better than the CTS, however.
The car does have a high price, but a less well equipped model would be substantially cheaper. The premium model has some equipment that more corresponds to the CTS-V, like the MagnaRide, Brembos and forged wheels, and then all the extra tech items such as CUE, front parking sensors, and the fancy safety equipment that's not yet available on the CTS. Comparable CTS models usually cost about the same as a 3 series, so a comparable ATS should really be about $5000 cheaper than a CTS, too.
I anxiously await the arrival of the ATS, especially the stick turbo. If it drives as well as or better than the BMW, I might even consider it to replace my CTS-V. I wouldn't mind a smaller car for the city with better gas mileage, as long as the handling, steering, and brakes are good, and if it's decently quick and fun to drive.