I've been using the Metro for a few months. The Metro system in DC is easy to understand, and I caught on to it right away.
The trains run fairly often and the stations I've gone to have always been clean and neat.
However, the train cars get super crowded during the morning and evening rush. They are especially crowded from 5 PM to maybe almost 7. Perhaps more people are taking the Metro now that gas prices are so high, but the Metro system should accommodate for this and send out more cars.
The Metro is also pretty expensive. The least expensive rides are $1.35. On weekends and from 11 to 3 PM you get discounted fares, but that doesn't save you that much money.
The Metro has very few stops and it is mainly concentrated in DC. New Yorkers will miss their subway since that has lots of stops and provides access to many parts of NYC.
If you can take the bus, I would say go for that instead. It is cheaper, has more stops, and it's not as crowded.
If you're going to be riding the Metro often, then consider buying a SmarTrip card. You put money on it and use it like you would a fare card. It costs $5, but only a few stations sells them. (Ask someone who works at the Metro.) I think that the card is worth the $5 because it is easier to remember and to find. The fare cards are paper and small so I lose them easily.
The Metro has a great website that helps you plan your trip by rail and bus. I highly recommend it: