I probably wouldn't do it again -- the National Express coach from Bristol is hourly, takes just over 2 hours and being direct is a lot easier than any train option, and usually cheaper if you can get a return. And on the way home, if your flight is delayed they don't charge you for changing your reservation to the next available coach.
I hate travelling by bus though, so if the train service to Heathrow was reasonable (in price and flexibility) I would use it every time.
Agreed ... we use the coach from Chippenham for Heathrow, which is useually operated by First even through it's under the National Express banner. The Chippenham and Bath service is less frequent and usually goes via the centre of Swindon, but by the time you've faffed around with an East-facing train service when you want to head West ....