I've done a few MegaBus runs over the past couple of years from Toronto to BUF and have generally positive impressions considering half of them were on the $1 fare (plus 50¢ booking fee). Biggest hassle is crossing the border, but that's no different from any other land common carrier, bus or rail. Only had one bad experience when the bus never showed at the Buffalo terminal and there was nobody in authority around, or signs about the delay or new departure time. The phone line was useless too, being on hold most of the time as my roaming charges added up! After half hour wait I bought a ticket on the Greyhound (also now has wifi) and headed for Toronto. I was out a buck or two so it wasn't worth my effort to follow it up.

So there is a drawback if you're boarding mid-route somewhere.

But on the whole, a considerable upgrade on the standard image of the inter city bus.