I just want to second (or third) the comments in support of DC in general. It has it's challenges like any big city, and it's share of neighborhoods to be wary of, but it's definitely not something that should strike fear into the heart of any traveler who's used to large American cities.

And yes, the Metro's not the NYC subway in scope of coverage or level of service, but nothing else in the US is. If you're riding on weekends, they do have a tentative schedule of which lines will see major closures up through the end of June: http://www.wmata.com/rail/trackwork.cfm

I've been visiting and riding the Metro regularly since the mid-80s. While the Metro hasn't aged well in some categories (i.e., maintenance), I have to say I've felt consistently safer on the Metro than most subway/transit systems in the US over the years.