I test drove the panamera gts, e63, m5, s6,and a gs350 fsport a few weeks back as I'm looking to get a new car for transporting clients and for grocery shopping and costco runs as my other car is impractical but very fun. I liked the panamera gts and s6 the most and m5 the least because the suspension and steering feels awful, but I'll probably end up with the gs350 fsport. It's a lot cheaper (and slower), probably a lot more reliable than the rest, and I already have a car for fun.
The car bmw should've made is the e60 m5 with the v10 and dct. Now that would have been a great car.