According to a Mintel report quoted today in the Economist, "Britons are far more satisfied with the service they get from their supermarkets than from their banks". The Economist draws from this the lesson that Tesco should get deeper into retail banking, competing to offer current accounts.
Of course the opposite is the case.
It's not that supermarkets are good at customer service and retail banks are bad, so that for example Tesco will be able to magically do well something at which Barclays and HSBC are failing (in fact, HSBC's First Direct won a top award for customer service only last year). It's that dealing with a retail bank, like dealing with broadband providers, public transport or most branches of the government, is unavoidably infuriating for many customers.