 I have owned my SL 550 for about 18 months so I have driven it in all conditions.  I have to say it is absolutely the finest car I have ever owned.   When the top is up, it is as quiet as they come.  I often conduct handsfree conference calls in it and people think I am at the office it is so quiet.  When the top is down, oh man, it is awesome.  The car is plenty fast in the SL550 trim but if you need a rocket ship, go for the SL55 or 65. But be careful, learn how to drive a car with that kind of power!   The ride is smooth and acceleration is excellent.  I live in a big city where the interstates are competitive.  I have all the power I need in this car.  I would highly recommend it!