I have test drove all 3 models of the ATS and I currently own an 2000 323i and I have to say they all reminded me of my car when it came to the handling prowess. My car does not have the sport package and rides on 16" wheels and the first thing I thought about the ATS when I drove it was how much it reminds me of the E46. The ATS is a better riding car but the steering, while good, isn't as direct as the E46. My car has all new suspension parts and I thought they were comparable on handing but the lack of larger wheels with better tires really doesn't help my car out. The ATS isolates the driver better from road impacts but otherwise they are very close. I have also test driven the latest non-sport F30 328i and it's softer than the E46 and the steering isn't as great as the ATS, but it has a great ride and I actually prefer the styling. The interior is a wash on either... the BMW is simpler and typical for the segment. The ATS has lots of different materials and textures but it looks upscale.
I'm not getting rid of my E46 for a long time but when I'm through with it I'm going to have a hard time choosing between the latest 3 Series and ATS. But I do have to say the spirit of the E46 lives in the ATS much more than the new 3.