No bank account or payment card details have been revealed (thankfully), but the logins (usernames/emails/password) could have been used to view users’ names, addresses and past energy bills.
2,200 accounts it seems have been compromised (at the moment) of the 14.7 million customers and British Gas have notified affected customers
The problem with these data breaches is those details could be used to gain access to other websites or used to send emails/letters pretending to be other companies trying to scam you.
How secure is my password? http://10ways.com/save/technology/how-secure-is-my-password/2508