A few years back I got my (ex) GF off of her 24 speed MTB on to a single speed MTB (Bianchi SASS). After the first ride she came into the house beaming with a big smile. She couldn't have put it any better when she said, "This is awesome! I don't have to worry about if I'm in the right gear. If I want to go faster, I pedal faster! If I want to climb a hill I pedal harder!"
She rode to work everyday after that.
Like others have said, pick the right tool for the job. SS/FG is great for most city riding. Most city riding is generally flat, paved, and only a few miles at a time. Perfect for a single speed. And for those wanting the extra workout over the short distance, riding fixed provides that. So you get a lot of "bang for your buck" during a 5 mile ride.
Then there's the maintenance. Many of us are comfortable with Park Tools, but most aren't. Each adjustment point an ANY bicycle is a point of failure. More points of failure means more things that can (and will) go out of tune. How many of you BMX guys remember dialing in rotors and 990s?
And to the normal rider, all of the gears look awesome, but awesome like the cockpit of the space shuttle.
Don't get me wrong. There is NOTHING like the feel of confidently ratcheting through a Dura-Ace corn cob and approaching then pushing the top speed that you can get out of you and your bike. But those long stretches of road of few and far between in the city.