Get what you want, don't listen to that guy. There are thousands of people that ride fixed gears in big major cities. Tell him to shut up and just install a front brake on it.
Riding fixed is a great feeling. I just switched over a few weeks ago from singlespeed and I'll never ride any other way again. I ride in NYC and it's just fine. It does take a few rides to get the coasting instinct out of your head so I would advise to ride around on low traffic streets and get use to it before you jump into heavy traffic.