+1 to the good thoughts in this thread. You can destroy your knees just as easily on a geared bike, by running the saddle too low, or grinding a bigger gear than your knees can handle -- I nearly did when I got back into cycling a few years ago. In contrast, if you gear your fixed-gear appropriately, use brake(s) to slow and stop, and develop good habits for riding out of the saddle when more leverage is needed, your knees will be just fine (and probably get stronger and more resiliant) riding a FG.
