Megabus is a division of Stagecoach, which runs bus services all over the country, as well as South West Trains, East Midlands Trains and the Isle of Wight's Island Line. (Stagecoach also owns 49% of Virgin Rail). It's neither much better or much worse than its big rival, National Express, which runs nearly all of the other express coach services throughout the UK (as well as some rail companies).
The only real difference between Megabus and National Express coach services is that Megabus has 'cherry-picked' the busiest routes (which are principally services to or from London) , whereas National Express operates many more 'cross-country' services. That means that Megabus can often (but not always) offer cheaper fares, but without the flexibility of routes (and scheduled connections) that National Express provides.
Chris