Megabus is awful. It's cramped (more cramped than Greyhound) and the buses I took were always very LOUD. The drivers on Greyhound usually say something before the trip if it's overnight about keeping quiet for people who want to sleep (like me!) but they didn't on Megabus. The PA was SO LOUD it woke me up and startled me a lot. It was much, much louder than it needed to be. About twice as loud as Greyhound's.
The prices are good; can be higher or lower than Greyhound depending on the trip.
One thing I do have to say about Megabus that's positive: I screwed up pretty badly once, and missed my bus (NYC - Toronto) because I messed up the time. Completely my fault. I didn't know what to do, so I talked to a Megabus employee at the "station" (roadside where they pick you up in NYC) and he said I could get on the next bus for $5 cash if there's room. I looked online and I guess this is legitimately their policy?
Sure enough, I got on the next bus. I was very relieved, grateful and appreciative.
But seriously, turn down your PA.