I have 18"s on my 535i, but I'm going to be selling them as the car just sits now. I run 17"s on my 540i, because I love the wheel style.
If you're in the SoCal area, I can make you a good deal on 6 - 18" wheels and 4 245/40-18 tires in almost new condition (<2,000 miles on them)...