This is my first MB and I'm in love; in fairness I've owned mostly Toyotas and Hondas so it's my first luxury ride but I'm very impressed.
After 6 months I've had no mechanical issues (though my miles are very low).
It looks great standing still and very very fun to drive.
Wasn't sure how much I'd use the convertible but I use it all the time, especially with the heated seats.
I mostly drive in the city but am getting about 20 mpg combined city/highway.
The ride and handling are awesome; it's quiet (even my cabriolet), smooth, responsive.
I love that I can manually shift the automatic from the shifter.
Audio is plenty strong enough for my tastes.
The array of features is great.
Visibility seems very good for me, esp for a cabriolet.
Looks and styling have held up well; I have a 2009 and for me the styling still holds up as classy and sporty.
The nav system is definitely out of date and suffers from the joystick navigation and poor interface in general - I've all but given up on using it. The iPod interface is quirky; songs show up on the dash display rather than the radio, and you use the steering wheel controls to operate it.
The two cup holders in the front and none in the rear are minimalistic. The driver's side mirror could cant out a bit further for me (I'm tall and drive with the seat fully back), but haven't had blind spot issues really.