I was sent some 20" wheels for my X5, but I had some tires laying around so I decided to try them out on my M3 before mounting on the X5 and I am really pleased with the look. 20" looks pretty nice on the M3, but can definitely tell the difference in ride with the super skinny 20" tires I have on.
20x10 +30 front, 20x11+30 rear Rota PWR
245/35/20 front, 285/30/20 rear
Let us know what you guys think