All the Ads for these seem to be pushing the 'Accurate Bill' line.
Many people who pay by Direct Debit end up in Credit on their account because the companies charge an estimated monthly amount usually higher than actual use. Will the Smart Meter based bill be an actual amount based on useage? Probably not. The companies want to charge over the top as all those extra amounts make them an awful lot of money. I always thought that one of the original ideas of Direct Debit was that it could be a variable amount, but its not in their interest to use this facility.
My bills are always accurate bills. I submit my meter readings online and get an Instant accurate bill.