I have found that the more I ride, the less energy I need to exert. I feel more alive, as well as more fit. Another great aspect of using a bike for transportation is the low expense. Though you have to dish out some dough initially, it pays off in the end. There are bike swaps and used stores if cash is a real issue.
Parking is hassle-free.
For security, I use a combination of a U-lock for the frame and a cable for my wheels. Although everyone I know has shared horror stories about bike accidents with me, I realized I just can't be afraid to get out on the street. Safety is a matter of prevention. I am both defensive and assertive on the road as a cyclist. This requires using my eyes and ears as well as keeping a good pace and taking a lane when desirable.