Can some DC residents clue me in on the practicality of owning a bike while living in DC as a method of travel? Are the sidewalks/roads conducive to casual bike riders (just trying to commute from place to place, not use as exercise). If I park my bike in a residential area or a Circle will it get stolen/vandalized? I would buy something used and fairly cheap from Craigslist, nothing fancy. Thanks!

DC is a good biking city.
Traffic can be heavy, but once you learn the routes you can find the low traffic alternates.
Some road conditions can be iffy (lots of new construction and infrastructure rebuilding), but most are fine.
Lots of excellent trails and mup's along with the roads.
Sidewalk riding is allowed outside of the central business district, but I seldom see the need to use them. The roads a generally safer and in better shape.
Generally flat terrain with short, steep hills as you climb out of the river valley.
Lots of people bike commute. I wish I could (car needed for work almost every day)
I know a few people living in the city or inner burbs who get anywhere they need to by bike, walking and/or Metro.
(btw - bike are allowed on Metro at non-peak times)

As for leaving a bike out overnight, I probably wouldn't unless it was really cheap, and never unlocked.
Daytime parking would depend on the neighborhood, where and how it was locked. I often lock up for a couple of hours and (almost) never have had a problem.
Type of bike is your preference, but for commuting I'd recommend something sturdy with good tires.

DC (and the surrounding area) are very bikeable. Check waba.org, bikewashington.org and bikepptc.org for some info. Also ask on the commuter forum - several DC area folks there.

The DC area is great for biking. We are EVERYWHERE! Join us.

CC is right though, lock it up tight or take it inside - it will get pinched in a DC second.

DC's a great place to commute by bike. I commuted between Prince Georges County and Alexandria for several years. It was faster than the Metro and about the same as driving, and alot more fun than both. I found the ADC Washington Bike Map to be quite helpful in planning my routes.

Don't leave your bike outside, though.

As far as whether a bike will get stolen, it really seems to depend on the neighborhood -- I know several bikes in my immediate area in Capitol Hill that just sit there (although I'm afraid that I just now jinxed them!). I bring my bikes inside, though.

Bike commuters, and people just biking in general, are pretty numerous and not unexpected. I can't seem to walk the block to the store without seeing somebody riding a bike. There are moments when I notice more bikes passing by my place than motor vehicles.