Hey BIGKEV176... I was worried about the same thing when I was looking around for rims. The car was riding so smooth on the factory wheels, I did not want to give that up for a shaky/bumpy ride. I had thought about only going up to 18", but someone convinced me to go with the 20". I have to say after going with the 20", I am completely happy. Mercedes does a great job with the suspension and that "air" ride, so honestly, I don't really feel any difference. The one thing where there is a slight difference is the noise. The 20"s with the Nitto tires are slightly louder... Again... it is only a slight difference. Infact, my wife can't tell the difference at all.
I am really happy I chose to go with the 20". I think it makes the car look great and I didn't give up any of that "comfort" feel.
Hope that helps...