I have been bicycling everywhere for a month now. It brings me back to my college days when I didn't have a car and had to rely on my bicycle and my legs to get from point A to point B and back again. Let me define everywhere:

Starbucks, grocery shopping, Costco, bank, library, errands, shopping, parks, parades, etc Nothing off road as my mountain bike is currently out of commission - I need a new one. All my rides have been less than 15 miles round trip through various road ways, highways, sidewalks, and very light trails designed to circumvent heavy traffic around town.

Pros vs Cons so far:

Pros:

Easier to fulfill some of my exercise needs without having to set aside a specific time for it.
Allows me to smell the roses and discover places / things I would never have noticed before.
Better for environment and traffic - overall public good. I actually get a wave and smile from people in cars.
During heavy traffic, I get to places faster than by car. In metro areas this benefit is magnified.
No $3+ per gallon fill up or waiting at the gas station to fill up.
I can go places cars can't.
No parking fees.
Access to the best parking spots, right in front of the location.
If something breaks, I can most likely fix it myself vs. car problems.
If needed, I can take my bike with me on public transportation to go further or get home.
Almost always beats traveling by foot only. I can jump off bike and instantly become a pedestrian.

Cons:

Limited range - particularly hilly areas.
Any accident with a automobile or truck can be fatal.
Carrying capacity is limited by what I can carry on my back. (More equipment needed).
Limited speed and roadway restrictions - some places really were only designed to be travelled to by car.
Possibly sucking up more pollutants on the road in heavy traffic.

Some upgrades and accessories I'm looking to get:

Some sort of rack / pack to mount on bike so I can get all the weight off my back.
More powerful lights when lights are out.
Pedal upgrade for more efficient power transfer.
Adjustable stem.
Seat upgrade.
Seat suspension.
Bicycle repair kit to carry for longer rides.
Better lock.