Although there have been quite a few replies I'd like to weigh in here since I live on bikes.
On what your bike says about you: That only matters on the internet, in the real world no one gives a shit except people who post on bike forums more than they ride. :D
On Fixies: The guy above who posted about ignorance is 100% spot on. Fixies are fun and good training, they won't blow out your knees if you gear properly and use your brakes, haha! I don't know that I'd use one as my only bike or my first bike, but it's an option. I'd advise SS for that.
On Singlespeed: Great option! And again you don't need to be a monster to ride one. My wife is not a hardcore cyclist but rides an SS 7 miles each way to play volleyball in the park then rides back, and yes there are hills. Start with an easy chainring/cog combo. Stand to climb and learn to rock back and forth with your whole body when climbing. Low maintenance and a more pure riding experience. You've got pedal and brakes, that's it!
On hybrids/gears: Nothing wrong with this either, gears are nice if you're doing extended climbing, want to go faster on flats and so on. I love my geared bike for my work commute. You should learn to basic maintenance since there is a little more to maintain. You might have to tune shifting, you tube is your friend.
This is all spot on. As a current fixie rider, I wanted to chime in as well. I used to agree with all the above posters about fixies being stupid hipster bikes not good for anything. Then I got my 21-speed bike stolen. I started looking on craigslist for a cheapish touring or road bike replacement. This being SF, fixies are everywhere and were half the price for a nicer bike in my price range, so I gave it a try, for a few reasons:
1) The aforementioned cost difference
2) I always looked down on them, but had never actually rode one, so it was an opinion of ignorance, so at least if I tried it it could be an informed negative opinion :-)
3) To learn something new. I love riding a bicycle, but when was the last time you actually had to learn a new cycling skill? After 30+ years of riding that opportunity doesn't come up often. The more I thought of it the more this appealed to me. And it is definitely a new skill and a new challenge.
So to those that say you need multiple gears for a bicycle to be usable: you absolutely do not. If we can do it in SF, you can do it anywhere. I wouldn't go on a long-distance tour on it, but for city commuting, perfectly reasonable. If you don't want to work hard on your bicycle, then get one with gears. Or an electric assist. Hills and especially headwinds are definitely harder, as you can't downshift and slow down, but have to maintain your speed to keep a comfortable cadence.
I will say if you're thinking about getting a single-speed, just get a fixie (or flip-flop hub). Same limitations, but more fun and challenging, imo. For heavens sake get a real bike with brakes though. I see idiots around here with no brakes because they want to be 'purists' or 'hardcore' or whatever and only use their legs to stop. I do get to see and hear some exciting stories of them careening into intersections unable to stop in time, or bailing into a patch of grass. You can still use your legs to decelerate if your bike has brakes.
Will I get another fixie when this one gets stolen? Not sure, maybe. Depends on what I want out of it at the time. I don't bicycle commute much anymore (due to overly crowded subway to get across the bay not being fun to cram a bicycle on, not because of the riding aspect).
