It is a little bit soft and disconnected from the road. I used mine for lots of highway driving (which is what I usually use my rentals for). Put about 2,000 miles on it in 9 days, and it can get 25mpg

. So while it might not be the best around the corners, personally I find in most of my US driving there aren't many corners to enjoy and a pillow is a good driving machine. To me it certainly feels and drives like it wasn't cheap to build. I wish I could say the same about the XTS I rented immediately after.
What I did like was the interior fit, finish and features. It has a nice headliner, padded grab handles above the windows (unlike the XTS which I'm sure has the same cheap bits as the Impala), a nice nav system, a front camera which I found was helpful in parking garages.
People renting cars have limited choice, and of that limited selection the K900 is certainly one of the finest vehicles I've had the pleasure of renting. I did enjoy the K900, more than any of the other luxury type vehicles I've rented over the years (528, X5, A6, Touareg, C300, G37, Q50, QX70, ATS, DTS, Old CTS, XTS, MKS Ecoboost, MKX, MKZ, Navigator, to name a few). If they actually spec'd out the BMWs nicely, say a 550 with all the options I bet would like it more than the K900. I expect I will like the new CTS if I ever find one when I'm picking up, but because of rental car duty I will be longing for a higher spec'd model (twin turbo). I also really like the Q50 because it's genuinely fun to drive, but it's about half as much car as the K900 and it shows.
My favourite rental in the fleet is still the Mustang GT (canada only) on a summer day. With Corvettes in the fleet I expect that to change. In my real not on the road life I'm drawn to CTS-V, BMW M, AMG, Tesla, IS-F, Audi S/RS, etc.