I pretty much adore the megabus as I don't mind long bus rides as much as most people. It's the cheapest way to travel if you book early enough and most recently I got a bunch of tickets for free during their "gofree" promo where they gave away thousands of free tickets.
I'm from new york and its the easiest way to get home for me from Pittsburgh, even though the bus ride is 8 hours long from Pittsburgh to New York. I usually sleep through the ride. It can be pretty uncomfortable and boring if you can't sleep during the ride however. I've never had a problem with the megabus other than that the wireless does not work when the bus is moving, which is pretty much all the time.
I'm not a fan of airports and getting to and from the megabus is much easier for me.
The key is to really just book tickets as soon as they are released, as the more tickets are sold for each trip on a route, the higher the price gets.
Pittsburgh to NYC peaks at $45, but I've gotten tickets for much much less and even free.
A downfall is that they only release tickets only three months ahead, so you can't plan a trip four months away usually.
My friends who usually take the megabus for DC routes say it sucks for them, I'm guessing the Pittsburgh routes are easier to maintain on schedule and such.
I'm hoping to go to toronto on the megabus sometime soon and have taken the bus from Pittsburgh to DC as well without any problems.