I have been a frequent user of National Express services between Bristol and London but I have recently switched to using Megabus. In a lot of ways I have found the vehicles to be more superior and on a number of occasions, the journey has been cheaper than National Express.
The Megabus staff at Victoria Coach Station have always been polite and helpful and ready to provide assistance when needed. I have also found the drivers to be extremely friendly and ready to assist when required.
A few points about Megabus which might be helpful are:
1)A number of the on street bus stops where Megabus services depart from don't have shelters, toilets,refreshment facilities or seating.
2)Megabus services can get extremely busy so expect to have somebody sitting next to you during the trip. If you get a seat to yourself treat that as a bonus.
3)A number of Megabus routes serve more than one destination and as such you might not get as great a choice of seats when boarding(if you are not fussy about a specific seating location then this is not a problem)
4)On two occasions I have been one of the first passengers waiting at the stop but due to having to put my luggage in the hold, other passengers without luggage have boarded ahead of me.
5)Some journeys are contracted out to other operators and the vehicles operated on these services do not run in Megabus colours so keep a look out for this when boarding. Such vehicles will have a Megabus sign and name of the destination in the windscreen
6)On the cross country route between Cardiff and Leeds, comfort stops do not seem to be offered. The bus stops at intermediate stops to pick up and set down passengers and then departs again. Therefore make sure you bring your own refreshments with you if you are going on a long journey.