Short summary: Megabus are one of a few companies operating low-overhead, curbside pickup intercity bus service. "On most city-to-city trips under 300 miles, the curbside bus offers tickets that cost a tenth of those of Amtrak and far less even than the price of the gas to get there by car." Passengers book online, tickets are delivered to cell phones, busses are equipped with WiFi and power outlets at each seat. Art…
It doesn't have to be an either-or situation, as both modes have their strengths and weaknesses. For example, while bus is cheaper than rail, it still has to deal with the same problems that automobile traffic has to deal with (weather and traffic delays being the most common). HSR is more expensive than bus, but is faster (talking about real HSR, not the Acela) and it's cheaper to add capacity.
That said, buses and HSR are complementary in lots of ways and there's no reason why they can't work together (just like no one is asking people to choose between buses and cars, we can have both). One example is the joint-ticketing arrangement between the Amtrak Downeaster (Portland ME) and Concord Coach Lines. Buy a bus ticket and miss the bus? Just jump on the next train (and vice versa).