Had this X3, my second, for around 3 months at 3,333 miles and the on-board fuel consumption average is 18.5 MPG.
My commute is 9.2 miles or around 18 minutes, about half of which is in stop-and-go freeway traffic that is sometimes also clear and at the speed limit in California; the other half is on streets in Emeryville, CA with stoplights, etc. My old 2004 X3 (E83) 3.0 averaged 16.0 so the new car is an improvement. This is close to the EPA rating of 19 MPG for City driving. Also I'm in the camp of liking the start/stop feature even though the savings may be minimal, and find that any jerkiness can be reduced if I lift off the brake to start the engine versus pushing the accelerator.
And, would hope to get 26 MPG if I did a longer freeway trip, and often saw in the low 20's when I checked my MPG on the computer in my old X3.
The new 2013 X3 takes some getting used to, you have to be more deliberate and the learning curve is higher because it can do so much more. I like it much more than my old X3 as the ride is much improved without sacrificing grip, the power delivery is a steady and more powerful flow versus a lunging sensation (which was good, too, just different), and the electronics put everything under your control. The iDrive interface can be managed intuitively without looking, navigation is great, and the increase in roominess and comfort is much appreciated.
It is less "engaging" than the older car because so much that was mechanical before is now electronic or computerized. The parking brake before I could initialize even if the car was moving; now with the electric parking brake I have to make sure I'm at a full stop otherwise I get a big jerk when the electric brake engages "right now."
This is all fine, just takes some getting used to. The steering feels less responsive than in the older car, which was also not responsive compared to my 2001 BMW 325i which has superb steering feel. But I can live with it given I now have 4-wheel drive and can't expect the front wheels to feel the same as a RWD car.
A very nice, grown-up car.