I just got a fixed hooptie through ebay a couple weeks ago. I was hoping it'd be close to ready to ride, but the headset and cranks were noisy, the bar's flexy, the brake's flexible and the wheel's got a suicide hub. On top of that, the frame's full of abrasions and rust spots and some pud engraved the same security ID on the downtube and the BB, just a couple inches apart, straight through the paint into the steel.
The gearing is 52x18 on 27" wheels and 170mm cranks. The seller simply removed the inner chainring from a ten-speed crankset, which incidentally is some bizarre Schwinn thing where the outer chainring attaches to the BB on a spline. Since the left crank has no spider (it's a mirror of the right crank), I can't swap out for a smaller ring -- I have to replace the whole crankset and possibly the BB.
So my new-kinda ride had to sit in the basement for two weeks 'til I could find the time to repack the headset (in decent condition, but it didn't have seals) and work over the drivetrain. It works mostly OK now -- after I resecured the cog because it unscrewed itself. This weekend I hit a bikeswap to get the parts it really needs, like a modern crankset and drops that are symmetrical.
So, why did I get it? Well, it's still cheaper than a frame and wheelset, even a crap wheelset. It's ridden to work and back, a 4 mile round trip. I have something I can turn into what I want, one part at a time, and still have something to ride in the meantime.
If you want practice playing mechanic and don't mind a ghetto ride, they can be a cheap way to try out a fixie. But you don't introduce your mom to computers by sitting her at a Linux terminal and making her build a GUI from scratch, and you don't introduce your friend to bikes by putting her on a trashmo. Get them on things that work unconditionally.