I would compared 5 and 7 series BMW's to Porsche products, except they have four doors. They are superb when rolling down the road, but the service/repair costs are staggering and build quality is marginal. My Porsche was precisely like that. Fast, attractive, and costly to own.
Mercedes is a little different. Not that an MB E/S is Corolla like in it's long term operating costs, but less than the BMW. You give up some "driving excitement."
My wife is hunting for a new car right now. We found a very cherry 540i, but she doesn't like the fuel economy. Then she found out the long term costs of owning one, and immediately striked it off her list. I would be more willing to live with the costs to have the driving pleasure, but not her.
Now, it depends on your context. We have always owned Japanese or German cars. We had a 1997 528i that we sold to our neighbour. She LOVES the car, and finds the operating costs low. She thinks it's deadly reliable. Talking to her, I was surprised at all the "minor" problems they've had, and was even more surprised that she was still happy with the car. BUT! Their old car was a Dodge Intrepid. They're comparing the two, and the BMW seems like heaven to them. If they had a Toyota previously, I'm sure they'd be singing a different tune.
I feel that the AMG products are the best of both worlds. Our C36 was as reliable as any other C-Class (which is pretty good) and could run with some pretty big dogs.
1998 C230 "Black Betty"