One sure-fire way I've found to spot a company or service that has unacceptably bad levels of service is that they have to put up signs exhorting customers not to assault their staff.
Coffee shops don't seem to need these signs, presumably because their patrons are basically happy that they're getting the coffee they've waited and paid for. Pubs, even though the patrons are often boisterously drunk, don't usually need to bother; clothing stores, computer hardware retailers, sandwich emporia, candlemakers, newsagents, mobile phone shops, internet cafes...none of these feel the need for signs asking patrons not to violently attack employees.
In fact, I see these signs almost exclusively in public transport providers and banks. Given that - purely on the evidence of the placement of these signs - the same people who are seemingly peaceable customers of almost every other retailer or service provider are expected to become violently enraged when dealing with banks and public transport, it's almost tempting to conclude that the problem is not on the customers' end.
I hasten to add that this is not (normally) the fault of the customer-facing staff, of course. It's quite true what the signs say, that they should - self-evidently - be free to go about their jobs without fear of assault or intimidation. But it is apparently a reasonable expectation amongst banks and public transport providers that they need to post signs asking customers not to attack their staff. Whenever I see such a sign, I wonder just how bad the service is going to be, that those providing it think it prudent to advertise the fact that it is likely drive me into a violent rage. And then, if at all possible, I try and find an alternative. Service so bad it'll make you want to hit someone is service it pays to avoid.