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!
Like most, the stereo and Nav system. That being said, I pretty much just use my IPhone now for navigation so that isn't as big a deal as it was in 2007 when the car first came out. Some entrepreneur should invent a smart phone/bluetooth interface for this car. They would make a fortune!