I finally replaced the chain on my fixed gear commuting bike, after 8 or 10 thousand miles on it, but I'd evidently worn out the cog also. The chain was making a grinding noise and riding up on the cog - I was mentally prepared for that to be the case but still a little disappointing. I've flipped the wheel to the SS side, nice and silent now, and I'll probably be OK with that until I get around to replacing the cog.