They are recommended for their quiet, quick routes and excellent accessibility. Subways transport more than three and a half million people a day. Therefore it is always up and running, regardless of date or time. The best way to handle payment is through the MetroCard. For safety reasons, avoid riding the subways in the middle of the night if you can, and keep your wallet in any front pocket or even in your hand if you have to.
Rush hour is an absolute nightmare, so bear that in mind as well.
All in all, the subway system is the recommended mode of public transport in New York City, for everyone, and the Transit Authority is to be applauded for their recent efforts to improve it. The MetroCard makes it that much more convenient.