I converted one of my bikes to fixed/single speed several years ago. Although I enjoyed riding it, the lack of gearing options was very hard on my knees and I had to reinstall gears. You learn how to ride differently when you only have one gear. My problem was that I found that I had to attack all of the hills (and my commute route is very hilly) and that led to knee injuries -- probably because my legs were not warmed up enough for that kind of riding while commuting. My knees started hurting within several weeks of converting my bike to SS and I stupidly did not relate it to the gearing change until I had damaged my knees, which took about 6 months of rehab to clear up (and they still aren't 100%).

However, if you commute in an area with flat terrain, I can see the appeal of riding SS/fixed.