I don't have a job so I don't have a commute, but when I did...
Pros:
I felt good getting some exercise.
The cool morning air. The smells of people cooking good food in the evening.
The beautiful ocean and mountain views. Seeing birds and the changing seasons.
Less stress than driving.

Cons:
Getting sweaty and stinky.
Having to carry a change of clothes or else having to ride slow enough not to get my work clothes sweaty.
Having it take a long time to get home.

Most of mine have been covered already, but I'll chime in.

Pros:
Commuting saves me a gallon of fuel a day.
Keeps me in shape (A friend keeps commenting on my ever shrinking beer belly).
I tend to get depressed, cycling keeps that at bay.
Gives me more reason to collect and wrench (Eg spend $$) on bikes.
If I get up early enough I can ride 80% of my commute alongside the Boise river.


Cons:
Time.
My car drive is 15-20 minutes, the bike ride is 45min-1 hour.
Cost.
Despite most things being a "buy once" type item, new bike parts and clothes for changing weather can be substantial.
Weather.
I don't mind snow, rain or sunshine, but a headwind is the bane of my existence after a long day.
Social pressures:
"So we're all hanging out tomorrow after work, you should drive so you can join us".

I save NO money commuting by bike, in fact it costs me MORE money to bike commute.

In summer, it's fun and enjoyable. In winter, it's still kinda fun and kinda enjoyable, at least better than train services where time delays are variable, whereas on a bike the worst that's most likely to happen is 3 flats in a row for which you prvosion 10 mins per flat.

I do it to save time later in the day. Before I commuted, I trained an hour a day. Now I commute, I save that one hour a day of training because my commute has become my training. Sometimes I'll bomb, sometimes a little more relaxed.

PROS
Save an hour a day in training
Dont have to put up with train delays
Dont have to put up with over crowded trains

CONS
Costs more money
Need to ensure everything on bike is packed in rucksack or panniers
Bike is currently parked 10 mins walk away from house (various reasons). Need ot unlock the 4 locks, load up the rack, change clothes etc. Shower, cant ride all the time in case need to go out in evenings. Cant wear shirt at work (usually wear jeans and top) because it gets creased.

New con...

-After poker night, riding to work hung over SUCKS!

I love everything about bike commuting and would say it is an integral part of my existence at this point .

Cons for me is the extra time needed to pack stuff up and a *slightly longer time getting to and from work....but not enough to make a difference or to change my behavior.

Pro
Riding on traffic free streets in the morning
Avoidance of wasting time in traffic on the way home
2.5 hrs of quality recreation without interfering with other activities
$ savings of gas


Con
Getting up early in the morning
$$$$$ spent on bike

Pros:
- It's fun when the weather is nice
- I find it relaxing. Get to unwind and decompress from work
- More idyllic way of commuting especially when going through parks
- Saves gas and transit cost
- Exercise is never bad. I lost quite some lbs since I rode!

Cons:
- Hate changing clothes. It adds to the "office time".
- Not liking hot nor cold days
- Risk of getting hurt by other vehicles
- Risk of bike being stolen.
- I prep more on days when I bike. Minor issue.

I ride everyday.

Pros:

Keep my constitution up with daily outside activity. (almost never sick)
Changing the intensity of the ride helped lower cholesterol with minimal diet change
Heart of a twenty year old @ 43 and 20lbs heavier than when I graduated HS.
No cars or transit costs.
...add not riding the rush hour steetcar petri dish as it's own pro as well.
Faster than both the above.. (retired messenger so commuting in traffic is an adrenaline sport)
Prescription free serotonin leveler, and other great brain juices.
Wife loves the muscle butt.
Winter riding is a blast.

Cons:

um....
uh....
I'll have to get back to you, don't have any right now.

Simple solution for #1 and #3 - add an electric kit. The purist hate to hear that, but since I've been using an electric bike (bionx kit), I don't need to change into bike clothes (except when it rains... then I need the rain pants) and I get there about 20% faster on the same effort.

Anyway, my pros and cons:

PROs:
- Great for environment
- Promote bike commuting by just doing it
- Save on car expenses
- Get a consistent workout
- No more looking for parking
- Seeing other people as people instead of boxed in cars
- Smelling the outdoors
- Reduces stress

CONs:
- Getting flats
- Changing into rain gear
- Lack of large storage
- Finding routes for new distinations (it takes time to find the safest route)
- No music, no more car karaoke
- Can't eat a Whopper and fries while moving
- Golfing and bike commuting don't mix too well
- Dangers of bad drivers (texters, drunks, cell phone yakkers, inattentive Whopper eaters)
- It just takes more time than a car for the most part

Pro: I get to "tweak" other people by riding in the street legally. (especially pick-up truck, truck, and Prius drivers)
I get 20 miles/burrito (undefined MPG)

Con: Other cyclists that think I may want them to converse with me while commuting.
They won't let me wear my shorts all day (Think "Sit down, Shup up" on Fox)

55 miles one way put a stop on my bicycle commuting.

And some of that would be done on I25 between Albuquerque and Santa Fe.

The train schedule sucks verga de burro.

There is no more buses, like I used to ride to work every day.

Pros:
More time on the bike.
More fresh air.
More (not much) money in my pocket.
About the same amount of time during rush hour.
Runs on local produce.

Cons:
Requires more planning.
Wears out the seat of my pants.
Can't carry as much.
Much slower at non-rush hour.
Emits methane.

You do know that most electric power is generated from either coal burning and hydroelectric dams right? Some are even nuclear powered too. Might want to think about it when you plug in your Bionx to the wall socket everyday.

Hmm... what fuel am I replacing? Food! American food - highly processed using lots of energy.

Even if you discard that arguement, there's another big plus for ebikes in regards to the environment.

There's been many occasions where I would've taken a car instead had I not had the bionx because I was either too tired, the trip would've taken too long by regular bike, my load was too heavy, or I had some sort of injury or illness.

I only need 350Wh to charge it fully to help me ride 60 miles. That's the amount you use to watch an hour and a half of TV on a 40" LCD screen. And compared to the additional car miles it replaces, it's drastically smaller.

Pros:

1. I don't seem to get sick anymore. And I have young children and am a teacher!

2. Did anyone mention that being in shape makes for better lovemaking???

3. More opportunities for meeting and conversing with a many more people than I otherwise would.

4. Most of what everyone else said

Cons:

1. Laundry

A lot of what everyone else said:

Pros:
-Combining excercise and commuting to work saves time.
-Biking to work allowed my wife and I to become a one car family
-The thrill of being "crazy"
-Biking to work let me quit a 8 cup of coffee a day habit.

Cons:
-I need to sleep more so I have less time
-Lugging around clothes. Especially when it is 30F in the morning and 70F in the evening.

Paul

Maybe in the suburbs, but I see many people in Minneapolis getting around on bike, even year round.

The last one for me, too. I just bought a LED flashlight to use as a headlight, and I was trying to find some colder weather gear (I found a bright orange Nomex jumpsuit), when my wife asked (pleaded) with me to stop. She doesn't want me to ride in the dark, or in the winter. I'm trying to squeeze in the last commutes I can in the next few weeks.
so,
Con:
Depressed when have to quit.

Pros:
The ride makes me feel great during and after it.
Keeps me healthy.
No worries about parking!
Being the crazy one who rides a bike to work.
The adventure. Riding under rain is damn fun.
It takes the same time to commute by bicycle or car, sometimes less.

Cons:
I hate, really really fking HATE all the arrangements that happen between the transition house > bike > work: putting clothes on the backpack in that special way so they don't get creases, putting it in a plastic bag so they don't get wet, putting the damn heavy lock inside the back pack, along with lunch, towel, etc.; riding to work with that ***ing heavy backpack; having a shower and changing clothes in a small and smelly bathroom (I specially hate that); doing it all again (except for the bath) when going back home.

I hate it so much that I almost gave up riding to work everyday. For those days I'm not in the mood I just get on my bike, have the ride of my life and when back home I put the work clothes and drive there.

Try a rack, panniers and mounting the lock to the frame. It'll far more comfortable.

Pros:
-Fun
-Exercise
-Traffic jams don't matter

Cons:
-Takes a long time, and that time adds up throughout the week.
-Really, really boring when riding home through the country at night.
-I hate having to change clothes.
-Having to spend extra time preparing for rain, snow, or really hot weather is time consuming and annoying.
-Can't just jump on the bike and head for a friend's house, the movie theater, or any store like I can with a car. Everything has to be planned out in advance.
-Sometimes driving is just more fun.

+1 :-)

PROS:
exercise
fun
saves money

CONS:
takes longer (I live in the suburbs, no time advantage with biking here)
inconvenient in some ways (can't carry as much stuff)
somewhat of an element of danger even though my commute is safe for the most part

I hear ya loud and clear.... bike commuting is very rare around here, and a lot of people just look at me like I'm some kind of kook, which I actually at times enjoy encouraging.
But wouldn't give it up for the world. Too many benefits... good health and fitness, cheap, don't need a license or insurance or any other of the kind of crap associated with cars. It's real freedom.

I disagree that you can't just jump on your bike and go to your friends place w/e. Could you do that when you were 12? Of course.