My wife's the worst case scenario for any car. She has a three mile urban commute, with about a dozen stop lights. The E82 got a 16.2 mpg average, and the E84 is averaging 17.4 mpg on the same commute. I think she's at about 4500 miles, and the E82 seemed to get more efficient until about 8k miles, so if that trend continues, things should get slightly better yet.
The three highway trips we've taken were done with less than 2k miles and returned right at 26.0 mpg, which is under the 27.5 mpg that the E82 got. I'm not sure if things will improve as much on that front, since the X1 sits higher and has awd. We'll at least run down to Austin soon, so I'll keep an eye on that once we make that trip.