First of all, congratulations to Sainsbury's. Even if it's a PR stunt as some have speculated, it demonstrates a sophisticated grasp of exactly how PR works in 2014. And if it's genuine (as I choose to believe), then they deserve recognition for the amount of liberty they give their staff to delight a single customer.
It's brilliant, so keep it up.
However, don't fall into the trap of thinking it's brilliant customer service. It isn't.
This is marketing. It's Sainsbury's showing off their brand values (or at least the values they'd like to be associated with) to the widest audience possible.
We're not anti-marketing, but we are anti-mistaking-marketing-for-customer service. Marketing is brilliant, and if you get it right (as Sainsbury's have done here) it will win you new customers and give a boost to the brand loyalty of your existing 'passive' customer base. But it won't get those customers to come back if they have poor or mediocre in-store experiences. It won't turn customers into super-fans who recommend Sainsbury's to their colleagues, and who walk an extra 200 metres (PAST a Tesco Local) to visit their nearest Sainsbury's.
… then no amount of marketing is going to save me as a customer.
The only thing that's going to keep my business is providing me a convenient way to give feedback (so I pick this option over walking out of the store and ranting on social media), reassuring me that my comments will be heard and acted on, and then making things right.
That's what creates super-fans who will stick with your brand through thick and thin, and who will recommend you to their friends – not marketing.
Want to know how to do customer feedback right and avoid the mistakes which make it ineffective? We've got a whole guide sharing the techniques of experts who've been there and got the feedback t-shirt.