What's the deal with fixed-gear bikes? All the cool kids ride them, but I'm not sure I want to lay out my cash for a bike that won't let me coast. Is it safe? Is it fun? Should I do it?
You're right; the Most Important Thing with fixed-gear bikes is, course, that you can't coast. When you go downhill, your legs will flail. If you don't have hand brakes, you'll use every strand of connective tissue in your legs to resist the pedals and control your speed.
The Wise Old Bike Guy is at a disadvantage here. Fixies weren't in favor (in northern Cali) back in my day; now that they are, I'm pulling my toddler in a trailer and conserving my rickety knees. Thus, although I've worked on a few, I don't have much experience riding them.
However, I can tell you this: Ride what makes sense for the riding you do, not what the fashionistas dictate. If you live up on Mt. Scott and work on Hawthorne, make with the derailleur gears and handbrakes unless you're really ready for adventure.
On the other hand: Fixies typically are inherently light and low-maintenance, great for improving your fitness quickly, and may even stick better on wet roads. (A driven wheel grips better, like in the Subaru (?) commercials.) They look elegant and classic, and they boost their riders' self-esteem by implying skill and daring.
If you do decide a fixie is in your future, don't skimp; plan on spending at least a few hundred bucks. Fixies are simpler than geared bikes, but that means there's less redundance for safety; the task of keeping body and soul together—in an emergency wheel-lock skid to avoid an SUV turning left across the bike lane, for instance—will fall to just a few 1.375" x 24 TPI threads that connect the cog to the rear hub. Make sure they're good ones.
That is, don't skimp on the rear hub, cog, and lockring. (For Christophe's sake, use a real fixed-gear hub with a lockring; don't cobble something together and "secure" it with Loctite or spot welds.) If you go with Japanese stuff (Suzue, etc.), make sure it has the NJS logo. This ensures the part has been certified to meet the stringent standards of Keirin, which is professional, government-regulated greyhound racing by humans for other humans to bet on. Beyond Japan, Campagnolo, Miche and Surly have good reputations. Use a big cog—18-tooth or so—so the chain will engage lots of teeth concurrently and won't slip.
As a fixie newbie, use a rim brake on at least one wheel; that will give you a bailout option while you learn to do downhills. Choose an easy gear combination, such as 40 or 42-tooth front to go with your 18 rear, so you won't strain so much on the uphills.
And oh yeah, don't forget a messenger bag in whatever brand the cool kids are using now. Does Prada make one yet?