Expedia works well if you don't have any issues. If your flight is cancelled you have to deal with Expedia which can be troublesome as you will have to rebook and get refunds from.them rather than BA.

It can be an advantage if you book a hotel at the same time as it will then be a package deal.

I would be inclined to use BA's best price guarantee and book directly if it's just flights you're purchasing in the rare occasion that you find a flight cheaper on Expedia.