Capital Bikeshare (or CaBi as many people call it) is great for short errands, one-way trips, casual scenic trips on the Mall and the Mt. Vernon Trail and for some bike commutes. Just be aware that downtown DC stations fill up quickly on weekday mornings and empty out on weekday evenings, during rush hours.
If you have a smartphone, download the free Spotcycle app. It provides near-real time updates on dock and bike availability. Use it to determine if your intended destination has available docks or whether you need to use an alternate station. Or use it to find if there are bikes available at your intended departure station.
I use CaBi very frequently, probably more than most people who also have their own bikes. I think CaBi is awesome. But I also understand the limitations in certain situations (mostly with bike commuting to downtown DC).