I’ve been with them for three-and-a-half-years and will stay with them until they do something wrong. Coverage has been excellent, generally, and the all-you-can-eat data saved me a fortune from needing a household broadband/phone. Also, my data allowance can be used in Ireland, where I travel regularly, and I can also phone/text back to the UK for free. 4G is rolling out in the NE now and speeds can be very quick, around 20Mb up and down.
I’ve needed customer/tech services a couple of times and found them to be very helpful. I even got a fiver discount n my bill because one if their masts was hammy for a couple of days, leaving me with intermittent signal at home until they sorted it out.
I’m always happy to recommend them, but would recommend getting a throwaway SIM card to test their reception in the places you visit the most. This advice would apply to any network you may consider moving to.