My road bikes have all been fixed or single free or fixed since i was 15, im now 36 going on 37.
Never had a problem with my knees, chains never came off (except once on a single speed commuter with a seriously cheap bent chainring on a roundabout in islington - ouch) and havn't always had a back brake as my main fixie was never drilled for one.
Used to ride with the south devon ctc every week on one, even touring round wales in my late teens with panniers and full kit - and ive always lived in hilly places, devon, peak district and now scotland.
Ride 52*19 at the minute, still riding on the same back wheel my mates dad built me 21 years ago. Got my first road bike with gears in 20 years recently as i inherited my granddads dawes galaxy, use that for commuting into the westerly gales up here in the winter.
I love riding fixed, you get used to it, its slow down hills, but it gets you fit going back up, im sure it'll fade as a fad, but cant see me ever getting rid of my mullett
love this bike to bits, cost me £220 2nd hand in about '87
(my mountainbikes got 27 gears though...couldnt do without any of em)