I drive a C-class Mercedes. For me, it was a combination of several reasons.
1) Brand name recognition.
2) Features. Can't really explain it, but having test drove the car it's head above anything japanese. The little convinient things, like retractable cup-holder, built-in self-diagnostics computer, 6 different reading lights, remote controlled windows and sunroof, built in garage door opener. Precission handling and responsiveness. But one of the most awesome things - whenever you drive any other car, you think "It'be kinda cool if they had this feature". With Mercedes - you have that feature. Even though it may seem not really important, you still have it, just in case. And having owned this car for 2 years - I can't drive anything else anymore. I travel quite a lot and therefore have to drive rental cars and it's absolute hell for me. I'm ready to cry from joy when I get back into my car.
3) Lastly - service. I got treated with respect and proffessionalism when I bring my car in for service. Needless to say - all service is totally free, including all oil changes.
Oh, btw - my previous car was Toyota Camry, so that's the car I used in comparison. During decission making process I test drove Mazda and Nissan. MB won hands down.