Never going back. Ever.
My details got hacked, and I was getting nuisance calls several days before they came clean about the hack. Customer service was poor; their systems don't fully support someone moving from one property to another and wanting to take their old number with them. I had to set up a new email address when I moved because my old one was permanently linked to my old property even though that account was now closed (hints at a very badly-designed IT system underneath). I signed up to a new 18-month contract and then they increased their prices 3 weeks later. When I rang them to tell them I was moving again I was told that the phone and broadband would remain connected at the old property (and therefore would be used by the new occupants) until the end of their mandatory one-month notice period on such things. Then they tried to tell me I wouldn't be able to take my number with me unless I used them at my new property (which turned out to be a lie). Just awful. I will never use them again. Ever.