I've used the Megabus a couple of times, out of neccessity. It's not so bad. I genuinely never have a good trip with National Express, on the other hand.
Heathrow is not what I was referring to. I was on about London. A well-priced feeder service from Leeds into Gatwick would tap into the ongoing low cost network from there (i.e. Easyjet).
Going by train onwards from KX to any of the airports in London for connecting flights is not particularly convenient, especially in peak periods and with a bit of gear to lug around. A well timed, well priced and well run air service down to Gatwick would make up lost ground over the train by virtue of the capacity to cut out London altogether.