I test drove a 2013 Audi S4 today, I barely consider it a test drive, the car had no gas (computer showing 0 miles to empty). My major concern with a car these days is the transmission and power delivery.
Generally speaking, I have not been a big fan of the flappy paddle gear box. I had an E60 M5 and did not particularly like SMG around town, it was/is great on the highway. I have also tested a Panamera Turbo of course with PDK (it was a much better 'here are the keys' test drive). Now that I think of it, I have test driven an M3 and 135i to test their flappy paddles.
I think the S4s DSG is programmed the best of any that I have driven. With the others, when you take your foot off the brake the car is still waiting for you to do something before it reacts, to me it feels confused and hesitant and then it reacts (and in some cases over-reacts). When you take your foot off of the brake in the S4 the clutch starts to take up and you start to move forward, it feels much more like a traditional automatic and like it knows what it is supposed to do. They are all fantastic at speed and on the freeway.
Power delivery: I would like to spend more time with it, but I liked the way the S4 made power. It is very smooth and builds in a linear fashion (which is my second favorite). The M5's V-10 has kind of screwed me up, I love the somewhat peaky nature of a screaming N/A engine its more exciting. I also had a 335i N54 auto, I didn't love the way it made power (not 'tuned' it was wifes dd) a little jerky off idle and runs out of breath up top. The S4 behaves more like a N/A then the 335 or the P-Turbo for that matter. One other thing, (again this may be the M5's fault) the S4 seems to run out of tach quickly, and when I wanted a downshift it was unwilling (maybe it would have caused an over-rev) or unable grant my request.
The thing that bumps me the most about the S4 is the variable assist of the power steering. At low speed it is so light and artificial it doesn't feel like the steering wheel is connected to anything, at speed it didn't seem to bother me, but like I said very short test drive.
I will test drive the new 335i or perhaps xi when it comes out, but for the first time in my life I am leaning toward the Audi (I have had more BMW's then any other car), but I don't like the direction that BMW is heading in these days, I suspect that I would like the Panamera GTS best of all but due to another recent large purchase it wouldn't be prudent for me to spend that kind of money this time around.