I run 91 octane during the hot summer months, or when I'm making an extended highway trip. Other than that, I've had no problems with using 89 octane the rest of the time. I notice about one mile per gallon difference between the two octanes, with my lifetime average at 19.2 mpg.

The one point I do consider important is the quality of the fuel itself. I run Shell gasoline whenever possible with Exxon as a second choice. I've had no fuel system issues ever, other than fuel level sending units.

2002 X5 3.0 261,000 miles
2004 325i 115,000 miles