No problems with my car yet. Handles better and is quieter then the old one. Oh, and a lot more powerful too...
The only things I can come up with are the following (if I REALLY had to nitpick):
I wasn't able to get + type DVD's to work in my COMAND system (burned with MP3's) even though the manual says it should work. Could have been the media and burner I used. Other people said that their manuals only stated that - type media should be used, so it could have been a typo.
Also, two flaps in the trunk were not correctly seated (or closed, depends how you look at it). Not a fault in the car, probably just an oversight from whoever built it. It's kinda like being delivered your car with the glove compartment open instead of close [

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But no rattles, no squeaks. Quieter ride with the top up, and water doesn't streak in through the top of the windows when I hand wash it. Break in was brill and the power comes up in gobs. I've accidentally reached redline quite a few times while playing with the F1 style 'switches' on the steering wheel. The nav system is neat, I don't use it much expect maybe to look at the map and get a quick idea of where I am. It's not a 3D style map, just an overhead, but that's a moot point. I don't think you can see the map well if it's some kinda 3D affair.
The 16 spoke rims are pain to clean, and overall the front spoiler is a lot more complex, so waxing/polishing/cleaning it (as well as the rims) takes time.
I have the option to open the roof remotely, and it too works fine. Uses Infra Red however, so you need to be in 'line of sight' of the door handles, but so far it hasn't blocked on me once (is smoother, faster and quieter then the R170 roof).
Interior space is a bit larger, especially laterally, I can feel it. Space to place my arms and so on.
And of course, the engine drinks a lot of gas [xx(] But only if you really push it [

] My TOTAL average is actuall 13 liters to a hundred or something like this, on the highway it's about 10 to a hundred. So it's almost the same as the old 230K.