OP - Thanks for this post/thread. It's always nice to hear about the experience from different people.

I have 19" in my 2009 35d and the ride is smooth and no road noise. We have cobblestone roads where I live and this thing just goes so smooth that you can barely feel any vibrations. However, those 20" just look AWEEEESOME. Every time I see an X5 with those fat formula 1 tires I go crazy. I might get them just for the looks. They make the car look so much more agressive. I do not drive much - I average about 5000-6000 miles a year so the road noise won't be much of an issue here. Have not pulled the plug on the 20"s yet but we'll see...