Megabus, for example, only travels between about 6 or 7 cities in eastern Ontario and Quebec. Being from western Canada, our choices are incredibly limited here - Greyhound between major destinations along the Trans Canada highway, basically, or a regional carrier.
That said, the bus is one of my favourite ways to travel, as I very much enjoy having the option to read, code, sleep, etc. on long journeys. This is the same reason I enjoy train travel.
What I really hate about them is that they have a schedule, but not really. They constantly sell more tickets than there are seats on the bus. The good part about this is that since your ticket isn't connected to a bus, you can take one any time on that day. But then of course 200 people try to get on an 80 person bus. It's a terrible mess.
To make matters worse, none of the other bus companies in Southern Ontario actually go to Ottawa, so its Greyhound or nothing.
So now I only take the train. It is always a pleasant trip. Definitely worth the price.
Experience on Greyhound: forced to buy two tickets for the same trip. Experience on VIA Rail: picture with the Stanley Cup.
That makes Megabus the Southwest of Canadian bus service.