I hate to burst everybody's bubble, but fixed gear bikes are not the end-all-be-all for workouts. USA cycling has done some tests on this. They found that track riders have the most inefficient pedal stroke - they only tend to pedal on the down stroke; this is becuase the rear wheel keeps turning the pedals over for you. So the conventional wisdom on fixed gear =s smooth pedal stroke isn't actually the truly revealed gospel, just a well known heresy.

On the plus side, riding fixed was found to develop leg speed, which is an overall component of your efficiency. If you can pedal faster under certain circumstances, you are more efficient (up to a point). Also, they were found to have better strength. This presumably comes from riding on justonegear. You need strength to accelerate, instead of changing gears to accomodate a constant output.

My road/crit racing success went through the roof in 2005 when I spent about 100 miles a week commuting on a fixed gear. I live in country that has some sharp little rollers that aren't so long. I had to adapt to keep the gear going and keep my speed up when cresting those hills, and improve my leg speed on the other side.

So I conclude that fixed riding can give you a HIGH QUALITY workout, under proper circumstances. That is, you are rested enough to keep your gear turning, have the right size gear and so forth. But it is my firm belief that to develop a well rounded level of fitness you should spend some time on a geared bike as well. That way you can work things like longer hills, jamming at very high speed on the flats in bigger gears, sprinting (on a gear bigger than a daily fixed ~75 inches).

Certainly I appreciate the love that we all have for the fixed gear. It will always be first in our hearts. But it does have its drawbacks and limitations from a training standpoint, just as it has its strong-points.