The X5M is as smooth and comfortable as it is fast. The handling is superb and it is an amazing machine overall. It is also very practical and I use mine as a DD. Yesterday I was driving in 2nd gear and went from 40mph to 60 mph in less than 1.5 seconds! All while being very comfortable.
The gas mileage isn't half bad. Mine has 7,500 miles on it, I got it in January 2011, and it's averaging 14.7 mpg.