I also made the same switch 4 1/2 months ago. The V was a very special car, to be sure, and did daily driver duty quite well. The reality is that the ATS is a better car overall to live with in real life! The V had virtually unlimited power, which I could never unleash on public roads, and I could never floor it. I rarely need full power in the ATS, either, but when called for it has been more than adequate, especially when on boost. A little less turbo lag would be nice, and I am considering a tune, but even stock it's been fine for the street.
I purchased my V more for the handling, brakes, and steering than for the 556 hp. The ATS competes quite well with the V in this respect. The V had an unbelievable combination of ride and handling, and the ATS Premium is very close. The V had a very slightly more precise feel and a tiny bit more grip, but I bet that if I put equally good tires on the ATS it might be just as good. I am considering the sway bar link modification too. The stock Bridgestone run flats are really better than I thought they'd be. The ride in Tour is pretty similar, though quite a bit stiffer in Sport than the V. It's stiff enough to be avoided on all but smooth streets, which are hard to come by where I live. The brakes are nearly as good on the ATS as the V, too. Where the ATS truly excels is in direction changes. Due to weighing 8-900 pounds less, there is much less inertia. Turn-in is RIGHT NOW. Along with the smaller size, it makes maneuverability in traffic much easier. While it doesn't look like it, visibility is better than with the V, too. I always had issues with the blind spots in the V, but none with the ATS, surprisingly. It all makes the ATS a much better city car, and I do live in the city.
Obvious other pros for the ATS include much better gas mileage, and thusly much better range. I would get 200-250 miles per tankful in the V, unless on a long trip, and the ATS is easily achieving 350-400 miles. No range anxiety at all! The interior materials are nicer and more durable on the ATS, and it's been rattle free. I do miss the soft padding on the dash and door tops of the CTS, but the piano black trim is WAY more durable than the V's, which scratches if you just look at it. I loved the Recaros in the V, but the sport seats seem to be equally comfortable and easier to adjust for more people, and are a lot easier to get in and out of. I also like the tech, overall, though I miss some of the simplicity of the older systems in the V.
There are some trade-offs, of course. I miss some of the options like the cooled seats, powered steering wheel adjustment, head light washers, and even the Ultra-View roof. The stereo in the ATS is pretty good, and has some nice features, but the sound was overall a bit better in the CTS, especially in the deep bass. The car isn't as roomy, and three in the back would be uncomfortable, where it was better in the V for sure. The V certainly has a bigger trunk, but the ATS has a split folding back seat if necessary, as long as you aren't traveling with four people (not available in the V sedan). The ATS won't get the respect of the V, either, if that really matters. My V used to get parked out front by the valets all the time, but the ATS hasn't had that compliment once.
Overall, I'm very happy with this car. Will I look at the ATS-V when it comes out? Sure, but I'm happier with this car overall than I was with my CTS FE3 in 2008. The V was a huge leap in excellence over that car. It will be harder to do that with the ATS-V, IMHO. Oh, and the ATS was only 2/3 the price of the V, and $5-6k cheaper than the equivalent 3 series, which I didn't like as much anyway. And so far, totally reliable, too. Seems like a keeper.