I have an LT this week (assuming it's a 2LT, though I haven't verified the engine). Overall I'm very impressed with the car -- it's a huge step forward both for Chevy and GM. The cabin is well laid out, and comfortably roomy -- I find it has more "elbow room" than the LaCrosse or XTS, both built on the same platform. Definitely a positive sign. The MyLink system isn't the prettiest system on the market, but it is responsive and easy to learn.
I can't say I really care for the aesthetics of the interior; the steering wheel looks like some kind of upturned pleather crab. At least on the LT, a lot of the materials feel cheap to me, particularly the "leather" on the seats and the dash. My car also has some obvious fit and finish issues on the inside -- for example, the panel at the center of the dash where a center channel speaker might go is raised by almost half an inch at one corner.
Though I reserve judgment for the LTZ trim, I'm going to go out on a limb and disagree with DrZoidberg -- I don't think this car is better than the Taurus, which I drove last week. It's a tough choice, but in the absolute, the Taurus has better driving dynamics, a more attractive interior finished in better materials, and a more enjoyable powertrain. I'd also give the 300 a slight edge, for better fit and finish, rear wheel drive, and the uConnect system, which I prefer slightly to MyLink.
Despite that, I'd take one of these again pretty readily -- though I like the Taurus better, the Impala is so much more spacious that I'd probably pick it anyway. I would take it any day over a Maxima, the LaCrosse, and the outgoing Avalon -- though I haven't driven the new one.
(Incidentally, I rode in a relative's new Hyundai Azera briefly over the weekend, and I'm anxious to see if any pop up at National. At least from the passenger seat, it looked like a really well appointed car.)