My wife hated driving her E-Class to work. Her colleagues were fine, but lots of raised eyebrows from parents. It wouldn't have bothered me a bit, but it did her. So, it depends on how you feel and the people you deal with.
I had no trouble meeting clients in my Mercedes, and few made any comment other than to perhaps ask me how I liked the car. Most of my clients were CEO/COO/CFO of mid to large sized firms, so a C or E Class MB was no shocker to them.
So, it's situational. A friend of mine wanted to build a super-eco friendly green house, and he heard of an architect that specialized in that type of building. The guy showed up to the meeting in a pimped out Hummer H2. Uh, you're marketing yourself to environmentally conscious clients and you're driving an H2?