I would like to address the knee issue. For as long as I can remember, I have had pain in my knees almost all of the time from running them to death as a long distance runner in my early and late teens. It is almost like I have no cartlige (sp?) left and feels like bone on bone pain. For years, physicians have been telling me that I could relieve some of the pain by building up the muscles on both sides of the knee. I have been commuting about 100 miles per week for well over a year now. For the first 6 months or so, I continued to have knee pain. Then I built a fixed gear with a much higher gear (76 gear inches) than I would normally run. I have been commuting on that since January or so. Even though I have ridden for years with the partial intention of building up those muscles, I really have never seen any real gains or effect until I forced myself to ride the tougher gear all the time. My legs are noticably larger and more defined. The knee pain is drastically reduced.

I argue that the "fixies and ss's will ruin your knees" argument only holds true if the rider does not capitalize on the true value of being forced to ride in one gear. That value is learning to climb much more efficiently by pulling with the hamstring much more than he otherwise would. Yes, one could do this on a geared bike, but I always just chose to shift down and spin instead. After riding fixed for one or two thousand miles, I am literally walking away from my old friends on moderate climbs and on the super steep stuff I don't fair too poorly either because I have learned an entirely different way of climbing that really is faster.

Bottom line for me....my knees are actually better off riding a single gear because I am finally building the muscle mass to support them.