We abandoned the Co-op/Smile when the hedge fund managers took over and have been with Nationwide ever since.
I have never had an issue with them on my own account (ha!) but I was going to go to town on them quite royally over their treatment of my Aunt Phyllis when I had power of attorney over her affairs. She was still perfectly capable of doing a lot of stuff for herself: ordering ready meals and the like, but when she enacted the PoA and they send me a debit card for her current account, they automatically stopped hers. I did a lot of research and discovered that they had already been hauled over the coals by the Ombudsman for this discriminatory behaviour (reports in the Telegraph).
The dispute was just hotting up when Phyllis fell over her coffee table, broke her leg, and never again set foot in her bungalow, instead coming to live with us. So the reason or the dispute disappeared and I was too busy dealing with her affairs and selling her bungalow that it became low priority.