The Chinatown Bus has become increasingly popular in the last few years. Tickets can be bought online or in person when the bus pulls up, and fares are amazingly low--around $20 for a typical one-way from New York to Washington, D.C. Safety records are hard to track, but the buses themselves are comparable in terms of comfort to what mainstream industry giant Greyhound offers, and GotoBus.com makes it easy to check schedules for all of the participating companies.
While each company provides similar service, however, the companies themselves are unrelated and do not accept one another's tickets. That introduces a bit of a dilemma for some travelers, because once you've got a ticket with a particular bus line, you're stuck waiting for that company's bus to arrive. On the other hand, the round-trip fare is about $5 cheaper than two one-way fares, money you might use to pick up dinner at a fast-food place before you leave.
Of course, there's no right or wrong answer here. If you'd like the flexibility to jump on whichever bus shows up first that's heading to your destination, the higher total fare is worthwhile. For those who want the best deal, buy round-trip and be prepared to wait a bit longer as other buses come and go. The important thing is that you're aware of your options.