Okay, i'll weigh in... I took my 335i E90 Xdrive to a fast paced event at NJMP Lightning raceway at the end of August this year. It absolutely blew some other cars out of the water - we were all beginners but i was flying by cars left and right that were purely RWD. This is my first AWD car, and it behaves like a RWD car but better on corner exits. I had more confidence than most when it started raining on the track as well. I had complete confidence when it was dry and just slammed the accelerator down to the floor past the apex and it just went exactly where I wanted it to go. My car is stock in terms of suspension/brakes which really surprised me how well you can take a corner at 80mph with your foot to the floor! I've had to turn traction control off only at one level because I didn't want to turn it off entirely. It surely helps to keep the car straight with some approved wheel spin. This has been the best all around car i've ever owned.
It was launched hard brake boosting at the 1/4 tracks as well. I video taped people watching me launch and their jaws dropped... its my family car, my daily driver, my snow plow, my weekend racer. What more can you ask from a car????
Here's a couple vids..
Forward this one to the 7 min. mark... they had an issue on the track and i couldn't stop the camera with my helmet on etc...