Here are my numbers so far compared to the X5 35d which I had for 11,000 miles. My 35i is still a little new to have completely broken in (3,000 miles).

City Driving Mix (50/50) - 22.9 35d 19.5 35i
Freeway 70 - 85 MPH - 24.5 35d 22.5 35i

The 35d was around 2 MPG worse during the winter on the winter diesel blends.

These numbers are averages for now, I'll have better 35i data in 6 months.