I needed to get down to LHR on Boxing Day a couple of years back & used national express - was amazed. OK, so I was with the missus so didn't mind her sitting next to me , and it was nonstop from the local coach station to outside T3 departures in just under an hour.

Would I take it again - well at a push yes. It was certainly way more convenient than taking the train (although the hassle of having to get from home to the bus station and then having to stick to their schedule does start to favour a cab if there are a few of you travelling...)