Here in Richmond, VA, the Megabuses--there are around 14 of the daily--stop under the I-95 overpass, in the parking lot between Main and Cary Sts., right across from Main Street Station. I'll admit that I take Megabus more often now than Amtrak when I travel from Richmond to Washington or New York. Comparing prices, an Amtrak ticket on 66 RVM-NYP starts at $63 low-bucket, whereas typically the Megabus costs me around $35, sometimes less if I book early. I'm poor, and traveling on Amtrak is something I consider to be a luxury.
Interestingly, this week, due to scheduling I returned from New York not by Megabus, buy by Greyhound. The ticket, bought online, cost me $41.99, one-way, nonstop. Now while the Megabus portion between NY-DC is typically full, on Greyhound, the bus was half empty and I got a seat-pair to myself. This was one reason I initially switched from Greyhound to Megabus: crowding issues. (Well, that and the surly attitudes of some of the Greyhound ticket agents,drivers, and baggage handlers towards their customers.) It leads me to believe that Greyhound really is losing customers to Mega/Bolt/DC Express/Chinese, which is helpful, I suppose, to the regular traveler looking for a little extra room on Greyhound once in a while.
I should add that whenever I travel, I typically only do so on about one-week notice. So I think regular Amtrak riders, at least on the NEC, know that Amtrak ticket prices definitely climb into the higher buckets within such a short period. Right now for a trip ten days away RVM-NYP, the coach price is $85, whereas the equivalent overnight trip on Megabus is only $27.00. There is no question of which one to take, for someone like me who is a serious budget restriction yet needs to travel frequently between these points. I can't afford Amtrak prices, and I can't afford to stay home, so I'll put up with a cramped overnight bus ride with a 3.5-hour late-night transfer in Washington in order to get to where it is that I have to go.