Great question. Aside from "dunno" I would have to say it was a deep want I have had since the first time I put a baseball card in the spokes. There is all the logical stuff. MPG, green, no insurance and all that. But really it is a 30 year old day dream.
The side effects are great joy, sense of peace. Meeting folks you would not have met with out one. Education in mechanics, math, physics, bicycle repair, electronics and a speed that lets ya see the road and your surroundings.
(and still cheaper then golf!)